Develop Reference

Building a hybrid app, where to start?

I know it's kind of an 'open' question to ask but I simply don't know where to start. Let me first tell you a bit of my experience/background so you know the level I'm at:
Build cd-roms with Lingo back in the days
Started web/front end development in the HTML3.2 era and kinda left off when XHTML was hot
Worked with Javascript 1.2 up till around 1.6 and later started working with Jquery
Actionscript up to 2.0, building websites and games
Did quite a bit of PHP/mySQL development. Build custom CMS' and later Drupal module development
All this was during internships or as a freelancer, everthing self thought by the help of books and such
So now I'm stuck with this idea to build myself an app. I've decided it should be an hybrid app as my target audience is very small but 60% is iOS and I don't want to leave out Android. Also I've been getting familiar with HTML5/CSS3 and started working on components for my app.
The basic concept for the app is that it's a layer on top of Google functionality: profile, calendar, circles and hangouts. Users will (eventually I hope) be able to create events for other users to rsvp to, they will be able to add eachother as buddies and chat about the events. A small step functionality wise but a huge step for me :)
I'm thinking of using the Crosswalk project for the Android version to counter the biggest issues around hybrid app development.
I've been reading up for the last months in preparation and the more I read the less I seem to know. Here's the stuff I think I should know more about;
Node.js
Gulp.js
Dev environment to work in (I once worked in Homesite, later Dreamweaver - don't laugh)
Packaging tools, etc etc.
Crosswalk project
Now I am not looking for some drag and drop software package to build an app in 3 days. I don't want to take the easy route per se, I don't mind a bit of a learning curve. I work on a Win8 machine, I know this isn't optimal but I can't change this.
Now where should I start? I was thinking NPM myself but you guys might think otherwise. Any good books on the stuff you think I need to learn to get me going?
Thanx for the advice!

I can share my experience with Cordova.
I work like full-stack developer and i'm quite expert to build responsive website, so the natural step to approach mobile dev was try Cordova.
First You can start to build HTML document totally responsive and try as webpage on your target devices.
A cool stuff, targeting mobile devices, is that You can use new CSS3 directive, without worry for compatibility with old browser.
First I installed Cordova following this tutorial.
https://ccoenraets.github.io/cordova-tutorial/create-cordova-project.html
Opening the folder /www on your project you can edit your app like a website, and you can also debug with the browser like a normal site.
To enhance the feature you will find a lot of plugins ready on github to extends the native functionality at the Cordova/phonegap project.
I suggest you also to build using onsen http://onsen.io/ a framework that let you use mobile UI easily.
Last tip that i can give you, if you target Android is try your apps on virtual machine, i use Genymotion https://www.genymotion.com/ that is much more fast then SD<
Good luck with your first app!

As you can imagine, there are a lot of framework where you can start.
Looking on your skills, maybe is better to use JS/HTML/css stack, but in this case, you will create an hybrid app in a webview.
In my opinion, webview applications quality it's only acceptable, but because of his productivity, in some cases could be the best decision.
Another good alternative is react-native. This framework allows to create a native application using javascript. React has his own markup language, is very similar to html. JS code is executed on native device using an embedder javascript engine, instead of webview, so, performances and experience can be compared to native applications.
And, at the end, my alternative, a framework I've made to write native UI and share business logic for each platforms. You can take a look on aj-framework https://github.com/bfortunato/aj-framework

Phonegap VS Steroids. Did I choose well?

I want to develop an App for Android ann iOS. It is a very simple App where there are almost no server connections and it is pretty simple. The only things is that it will have many animations.
I thought that would be a good idea to choose Phonegap instead of a native application because it is multiplatform and it is very much easy to develop for a web developer like me.
But, when you read in the web you see a lot of people who say that it is really slow, that buttons doesn't react inmediatly and more things.
So first I would like to ask, Will the phonegap performance be enough for this kind of App.
And secondly, I have found Steroids. It looks great because it takes Phonegap and make it native. But it sounds too well, I mean, you have all of Phonegap with more performance, I guess it should have some problems.
So, could I use Stereoids instead of Phonegap to increase the performance? What it worse of Steroids?
Greetings and thanks.

Full disclosure, I'm an AppGyver employee and am of course inherently biased towards the Steroids platform, but here's a quick rundown of why you should choose us over PhoneGap:
Theoretically, anything that can be done in PhoneGap can be done in Steroids as well. In reality, there are a few lingering problems with some custom plugins that might require some tweaking to work through our Build Service, but core Cordova plugins are supported.
Steroids provides Native UI features to make your app virtually indistinguishable from an actual native app. We've gone to great lengths to ensure that the end user would not be able to tell a Steroids app from a fully native one. This includes eliminating the delay between button taps and the app reacting, as well as providing ready-made implementations for drawers, modals, tab bars and all the goodies that you would find in a native app, but have to build by hand in a PhoneGap app. Admittedly, this feature set is currently only available for iOS, but our Android support is catching up fast with the first UI features being in internal QA at the moment and more coming soon!
Our qr code system makes demoing (and developing) apps insanely fast compared to having to constantly rebuild and install demo versions of your app. Just download the AppGyver Scanner, generate a share code and scan, and your app is now running on your phone.
We provide support for a Chromium-based Android client, which blows the old Android WebViews out of the water in terms of performance and cross-platform compatibility. Once again, this is still a very young property, so full support for all Steroids features is not there yet, but Android is currently our top priority, meaning things will improve fast.
I am glossing over the details quite a bit here, but this should give you a general idea of what we feel makes our product great!

Struggling between native and phonegap, simple app requirements

I'm going to make a native (meaning; not in the browser) mobile app. Since I'm a web-developer I'm struggling to decide whether or not I should try Phonegap or just build an native app in java or objective-c.
The app requirements are simple. GPS/wifi location, Facebook integration and I guess I'll need a database to handle some of the application specific Facebook-friend relations. Like the highscores in a game, for example - stuff like that.
I'm a web-developer, and don't know neither java or objective-c, yet. I've never used Phonegap before, so I don't know if it's capable of fulfilling my requirements.
So my question is as follows;
Can I use Phonegap for my app, or do I need to dive into a new language?

Related:
Larger version of this article can also be found HERE, to be transparent it is my personal blog.
Hybrid vs Native apps
Hybrid apps:
Pro:
Quicker development, especially if you're a a longtime web developer.
1 language can be used on every available platform. This, of course, requires a Phonegap wrapper. You don't need to learn Objective C, Java, or C# to do any development. You need only to understand the basics of the Phonegap implementation. You will need to play with above-mentioned languages a bit but you don't need to understand them.
Phonegap can offer some native capabilities like iOS native tab bar or Android native tab bar and so much more.
Lower budget costs and a huge community of supporters and developers.
A hybrid app offers many of the advantages of both approaches access to the most common device APIs, and broad device coverage while not requiring the specialized skills, bigger budgets and longer time to market that are more typical of fully native apps.
Cons:
You will piss blood after some time, literally and metaphorically.
Mobile phones (even today's tablets) are not fast enough to smoothly run a hybrid app, mobile JavaScript capabilities are bed at best. Android platform is a nightmare, page transitions don't work smoothly not to mention lacking CSS/CSS3 implementation. If you think native Android 2.X and 4.X have differences take a look a Android 2.X bad JavaScript / CSS implementation. iOS fares better but still has a lacking CSS3 implementation thou much better page transitions. To make this short native apps will always have a better user experience and general feeling.
You will spend much more time fixing the app then building it. Creating an app for each and every platform is a pain in the neck. Browsers on different platforms do not uniformly support all the latest HTML features and APIs, which can make developing and testing a challenge.
If you don't have a good designer, don't even try to build an app; looks are everything.
If you don't know what are you doing there's a good chance your app will not get permission for Apple app store. Even Google Play Store will ban your app if they discover a 3rd party Phonegap PayPal plugin.
Native apps:
Pro:
A native mobile app can produce the best user experience — fast and fluid, can give you the best access to device features, and can be discovered in the app stores.
Without a doubt, native apps have full access to the underlying mobile platform. Native apps are usually very fast and polished, making them great for high performance apps or games. This is more then enough.
Cons:
Bigger budget, you will need at least a person/s with Java and Objective C knowledge, even C# if you want a Windows mobile app. Yes you can learn it yourself but don't live under the illusion you will do so in a short time. If you have never done any development, choose Objective C (iOS is still a better platform). In other cases choose Java. Java has a syntax similar to other available languages, so it is easy to jump from C# to Java and vice-versa. Objective C is a world of it own. It has a rather uncommon syntax.
It might take you a longer period of time to develop all of them and time is money. This depends on the complexity of the apps.
In your case, if you have a good web development/design skills and/or have a good designer you should choose a hybrid app. Go native only if this app is going to be a complex one. Everything you want can be done with a hybrid app. And you will find a great and supporting community.
Hybrid apps
What I didn't previously mentioned you need to be specially careful here. Even if you are seasoned developer you will find a lot of problems you can't solve. Hybrid development should not be mistaken for a basic web development.
Every mobile platform has its own set of problems, not to mention that mobile phones behave differently depending on a device. If possible always create a app that works on a slower devices because native app will successfully work on almost any device.
There's one last thing, when creating hybrid apps, app development will take you about 30%-50% of your time, rest goes to specific platform debugging and learning. Believe me, even after several years of mobile development sometimes I am facing problems I can't solve alone or fast enough.
So be careful when choosing mobile framework, pick 2-3 and spend several days reading about them.
Links
Here are few links to help you decide:
PhoneGap + jQuery Mobile 1.4 tutorial - This is a step by step tutorial for PhoneGap and last version of jQuery Mobile
PhoneGap + jQuery Mobile 1.4 tutorial - MacOS version - This is a step by step tutorial for PhoneGap deployment on MacOS. jQuery Mobile part is covered in a previous article, no point in repeating things.
jQuery Mobile vs Sencha Touch - jQuery Mobile Sencha Touch comparison
jQuery Mobile vs Kendo UI - jQuery Mobile Kendo UI comparison
7 best known HTML5 mobile frameworks - Name tells everything
7 less known HTML5 mobile frameworks - Name tells everything

if your application does't have animation effects and simple i prefer to create in a web(Phonegap) cz at less amount of time we can launch in multiple types of mobile OS.
if your app contain more UI effects and animation then it is best to go with native..
even though you develop in webview(Phonegap) you need learn some basic steps in Android and iOS
How Phonegap performs in devices
Ios
The animations and view loading in ios webview is stranded and good, the animation effects can observer clearly
Android
We have to consider the device version and device company for android while running the html code, Some high config devices in android give more clear and efficient animation display, but some devices (mostly old version and some company devices) cant give good animation effects, rendering effect varies from device to device in android,

In my humble opinion with Phonegap you can just develope simple apps, nothing more. The documentation is not enough and like others have said you will spend much more time fixing bugs and finding plugins than building your app. I would make the effort to learn Android or iOS or whatever, look at it as an investment of your time. (Yes, my comment is a little biased).

Last I looked into it PhoneGap was all html,css, and javascript. So, you may feel right at home. PhoneGap can handle GPS location, see this article from PhoneGap's documentation. Whether or not you want to pick up Objective C would depend on how fast you pick up new languages as it is a bit of a departure from web development.

It totally depends on your needs
After publishing my first app several months (native Android, database driven, restful connecting app), I decided to rewrite my app by using phonegap (since I want to clone the app to iOS, so instead of doing it in ObjC, why not javascript ?)
When on Android, it costed me, a Java developer, 3 weeks to learn and code everything from scratch. Banging my head to the wall several times for how to make the ListAdapter sync with the Restful Service from my server, how to theme the app (quite hard in my opinion, especially when it comes to special effects like transparent, border,...), and also, how to structure your code base effectively (if you are familiar with Java, you will find that all the classes are very coupled to each others, since that is what most of the tutorials write about)
When on Phonegap, everything is done in 1 week. Css, html, with onsenui and angular make the GUI development just like about 1 st year student 's assignment and theming is just a piece of cake. I don't need to run it on the real device, just use browser during the development, change code and refresh are enough. Angular provides 2 way binding with the DOM, and this makes a huge awesomeness when collecting and showing data. For example:
<ul ng-each="for sentence in sentences" >
<li>{{ sentence.title }} </li>
</ul>
When the variable sentences is changed (retrieved from server), the DOM is automatically changed as well. However, javascript is only javascript. I found myself struggling many time when debugging the app, especially when doing the MapReduce functions with PouchDB (big mistake, have to go back to cordova-sqlite-plugin tonight) and most of the time, I have to read the log in order to figure out the problem (Chrome 's tools and Firebug cannot catch the breakpoint). Also, it is hard to restructure your app (javascript, again) so if you don't organise carefully your code base at the first sign, you might end up with a mess (and debugging is a real pain, again). PhoneGap is also Web based, so sometimes, you will hear that "because on this platform, it does not support that bla bla bla" (e.g, localStorage) and you have to stand for it if you cannot find any good plugin.
Conclusion Except for the limitation of the web-based storage, PhoneGap works quite well with the database driven, simple GUI app (and most of the database driven apps currently has a simple GUI anyway). If you gonna develop that kind of app, then stick with it

check out using ionic, backed by angular.js to offset some of the cons mentioned in the accepted answer. Tutorial for launching an employee directory app: http://coenraets.org/blog/2014/02/sample-mobile-application-with-ionic-and-angularjs/

#Gajotres wrote a great response, but hybrid app performance has come a long way since 2012. I answered a related question that covers the difference between hybrid and native app development in 2017.
The moral of the story is that hybrid app development has improved tremendously in the past few years, but it still cannot match native for certain use cases.
With tools like Ionic (first released in 2013) and React Native (first released in 2015) it is easier than ever to build hybrid apps with native-like components. Hybrid apps are now faster, less buggy, and able to interact more deeply with the native UI.
The threshold for hybrid app performance is substantially higher than just a few years ago, but there is still a point at which you may need to develop your app in a native language.
Original Answer:
Yes, the general sentiment is that Hybrid apps are inferior to Native
apps. While this can be frustrating for developers more familiar with
web technologies, it does come for good reason:
Inability to interact with native components: Although plugins such as cordova-plugin-statusbar exist, there are limitations
interacting with and manipulating native components using web
technologies. One great (and frustrating) issue I have personally run
into is the inability to have an input at the top of they keyboard as
the keyboard animates in. This sounds like a nonissue until you look
at an app where this is an essential feature such as in a chat app
like Slack.
300ms delay: Although modern browsers are beginning to phase this
out,
the fraction of a second delay present on Hybrid apps makes the app
feel slow and non-native. This issue is becoming less of a factor as
more users adopt workarounds such as
FastClick.js and some
frameworks such as Ionic eliminate it
by default.
The haters are right (sort of): While Hybrid app development has come a long way, there are still minor glitches and laggy
functions that are just not present in a Native app. Screen
transitions, app switching, and battery life are still common areas
for bugs to appear and likely will be for some time, even if they are
starting to become less and less noticeable.
There are some great Native solutions: With newer languages such as Apple's Swift it is becoming easier to
code in a Native language. That being said, tools such as React
Native fall into a gray
area between Native and Hybrid by allowing developers to code in
friendly technologies such as JavaScript but compile into native code.
The moral of the story is that it really depends what is important to
your specific use case. Hybrid apps have become a viable option and
are no longer an embarrassing side show. Conversely, there are still
minor aspects of interacting with the Native UX that are not yet
possible except with a Native app.
Overall, I recommend mapping out your project and determining if your
app needs any of the benefits of a Native app. With tools such as
Ionic View app it is easy to put together a
basic mockup of your app and test on a real device whether or not a
Hybrid app will work for you.

There's also AppGyver Steroids that unites PhoneGap and Native UI very well.
With Steroids you can add things like native tabs, native navigation bar, native animations and transitions, native modal windows, native drawer/panel (facebooks side menu) etc. to your PhoneGap app. Basically, anywhere CSS + JS don't cut it, Steroids reaches into the native layer to perform the action.
Also, with steroids, you can (although you don't have to) use multi-page architecture (multiple webviews) rather than single. With single page apps, all things like transitions are just CSS workarounds, and as Mani said, even modern tablets and phones aren't fast enough to make them smooth. It's always obvious when it's a phonegap app, especially when you've got a bunch of processes going, or if you're scrolling since JS processes are haulted (being fixed in iOS 8).
Using the multi-page architecture means you get 100% native performance (because those parts actually are native). And unlike phonegap, you'll get consistent user interface behavior and easier debugging (with a single page app, it's often difficult to recreate a particular state in your app). Multi page makes it far easier to develop and maintain your apps since your codebase stays cleaner.
Check the demo: http://youtu.be/oXWwDMdoTCk?t=20m17s

I also find a every good article here http://www.comentum.com/phonegap-vs-native-app-development.html, it compares these two ways in different categories.
The summary is as below:
Design of Interfaces (Same)
Cost (Hybrid has Advantage)
Development Timeline (Hybrid has Advantage)
User Experience and Performance (Native has Advantage)
Maintainability (Same)
No Limitations (Native has Advantage)
Security (Same)
Support and Resources (Native has Advantage)
Tools and Debugging (Native has Advantage)
Platform Independent (Hybrid has Advantage)
Popularity (Same)

One overlooked of the great options here is development with Xamarin.
Though it doesn't help to utilize OP's web developer skills, it has many pros compared to PhoneGap and almost no cons.
Xamarin vs PhoneGap Pros:
Mature framework providing access to the most (if not all) native
features of mobile devices.
Learn only one language (C# for the most developers) for all mobile
platforms.
Have a performance of native apps on iOS/Windows Mobile (since app is compiled into the native code) and very close one on Android (it still compiles to a native APK, but has to have a thin wrapper that translates .NET class library API into Android classes API at the runtime).
Maintained by Microsoft which supports the overall generous quality of the
platform itself and development tools. For instance, you will use Visual Studio and it's probably the best IDE around and now has free Community edition with every feature available that you'll need for Xamarin development.
Xamarin vs PhoneGap Cons:
Needs some time investments for pure web developers to switch to C#
and Xamarin class library API.
Therefore, Xamarin is a very solid option for cross-platform development nowadays.

Appcelerator vs Android SDK

I have been looking at appcelerator it seems pretty fine! Without a doubt, one of the advantages of appcelerator is its support for multi-platform. I am interested in building an android app and maybe a iphone app later on. So it is not crucial to support multiple platforms at the moment.
If you disregard supporting multiple platforms and just focus on android development. Is appcelerator still advantageous? Does it lack any features of "Android sdk"?
(When I say "Andorid sdk" I mean development with Eclipse with native Android sdk, if it makes any sense)

We looked at Appcelerator when starting our Android project. We knew we would have to do an iOS client next, so Appcelerator was enticing.
We decided not to go that route because it doesn't support all the native features of each device.
For example, we decided early on that our user interface in Android would need a widget. No support for that in Appcelerator, as it's specific to Android. [This was a few months back -you may want to check again].
Compromising on the user interface was too much for us. We decided to go with C/C++ for the app logic and use Android SDK for the user interface.
Our apps now have two pieces: C/C++ as much as possible to the "brains" and the native (Android/Cocoa Touch) for the UI to take advantage of the UI experience.
Works of course for platforms that can integrate C/C++.
The parting advice: design your user interface first, then find a tool that can implement it. Users have high expectations of the UI in portable devices. Compromising on it early on may be the KOD for your project.
[EDIT] Every so often I see an upvote for this question. I would like to update with what we learned in the past two years:
Using C/C++ for the common parts of our application has paid off. It does require a slightly more complicated build process, but the savings are gigantic if the piece of code being shared is complex (as in our case).
For the user interface piece we are beginning to look into hybrid apps (some UI elements in HTML). There are still debates out there about HTML interfaces (Facebook and LinkedIn are two that move to native code for the UI), but also some reports that when chosen carefully it works. There is a great talk from Flipoard on that; with slides here.
UPDATE Oct/2014
In March/2014 Smashing Magazine published an excellent article comparing native iOS, native Android, PhongeGap (Cordova) and Appcelerator Titanium. They show the development of a simple app in each environment.
This is the last part of the series. At the top of this part there are links to the previous parts of the series and at the bottom there is the comparison of the approaches. There are also interesting comments from readers at the end.
UPDATE May/2015
Still get an upvote for this question every so often, so I would like to share what we have done since I wrote the first part of the answer.
We are now working on a project that also has a web client. We have now the iOS client, the Android client, plus the web client.
JavaScript is the natural choice for the web client.
Since we want to share as much code (at the business logic layer) across these clients, it means we need to find a way to run the JavaScript code in iOS and Android.
And that's what we ended up doing.
This is a summary of how we handle it:
For all clients: all data structures are defined with Google's protobuf. This allows to automatically generate the serialization/deserialization code for iOS and Android.
iOS: run the JavaScript code with the help of JavaScriptCore (a good overview here).
Android: run the JavaScript code with the help of Google's V8 engine.
The bridge between JavaScript and the iOS/Android layer can be a bottleneck, especially the serialization of the objects. We had to optimize a few things and learned a few lessons to stay out of trouble.
Generally the experience has been positive. We saved significant amount of time by sharing the code across all clients.
If I had to start another project like this one (that requires sharing code with web and mobile clients), I would also take a look at what Google did for inbox. It wasn't available when we started. Looks promising.
UPDATE August/2015
And the world keeps turning...
I would also take a good look at React Native if I had to start a new mobile app now.
It's based on JavaScript, bridging web and mobile development more easily.
There is a great tutorial in Ray Wenderlich's site.
UPDATE February 2016
My shortlist for cross-platform development is down to two:
ionic
React Native
I've been paying more attention to ionic because our web app uses AngularJS and so does ionic (Cordova + AngularJS). React Native will force the team to learn another framework (of course the argument is the opposite if your web app uses React).
Nevertheless, I'd seriously consider React Native, even having to learn another framework, because of the reported performance and look-and-feel of the app. From React Native's site:
With React Native, you can use the standard platform components such
as UITabBar on iOS and Drawer on Android
If you are starting now, I recommend you take the time to at a minimum complete their tutorials so you get a better feeling of each framework. Each tutorial can be complete in one to two hours:
ionic tutorial
React Native tutorial
For a simpler approach: Smashing Magazine just published (at the time I wrote this update) a good article showing how to use the native navigation elements to make the app feel (well...) native, while using web views to render the content. The article goes into the details of how to make the HTML/CSS usable in the mobile devices (it's not just pushing the HTML/CSS you already have).

If you only focus on develop apps for Android platform, I recommend you should use native Android (Eclipse + Android SDK as you said) instead Titanium.
Reason: Titanium is suitable if
You want to develop apps quicly, or
Your apps is simple, or
You see Java is so difficult (maybe :p ), so you choose the easier (Javascript of Titanium).
And here is the reasons why you should use native Android instead Titanium:
You want to develop a complex app, or
You want to customize app appearance. It's very important. Imaging you make an app for client, and they need their app to be customized for more eye-catching. Choosing Titanium means you must use their control, which lacks of ability to customize. It'd be better if using native Android that you can customize everything you want.

that is a very difficult question to answer without any specifics regarding the requirements of the application.
I would suggest you take a quick look at the API documentation of Appcelerator to see what the framework provides and also take a moment to list the basic requirements of your application and then determine what tool will be best for your project.

I think you are onto a good start using Appcelerator, since it uses JavaScript, etc. Easy to learn, but good examples. It will also enable you to use the same code and reach iPhone,iPad apps. Learning to use just the Android SDK is more complicated. Even then you still need to develop later iPhone etc apps. If you come from a web background this makes sense. There are good video's for quick learning. Good Luck

My experiences with Appcelerator seem to suggest they are more of an iOS shop. Their toolset for Android is, IMHO, quite lacking. ADT supports visual UI building and debugging on device, which Titanium Studio does not. There has been a bug filed about this for several months now, and it continues to be delayed. If your focus is Android, use Google ADT or MOTODEV Studio for Android. These IDEs are quite nice and are used by professionals.
The above answer presents a great way to develop mobile apps. DO NOT COMPROMISE ON A HIGH-QUALITY USER EXPERIENCE.

Lacking of bluetooth support is a huge deficiency for Appcelerator and no one gives a clue about roadmap.
edit : appcelerator now have BT support in Tizen 3.1

Well, in the new version of appcelerator (APPCELERATORSTUDIO 4.0) you can have alloy projects, and something like css files for your UI (tss), so in fact they are really customizable.
Greatings

Appcelerator has a lot of support and it really easy to use. It supports 100% of native api calls now. They have really good documentation too.

developing smartphone apps using Rhomobile

I have been developing an Android application for about two months now, and the guy I'm writing it for wants me to use this instead of the android SDK so we can deploy the application for multiple smart phones: http://rhomobile.com/
he says you can write the application in one language and it can be deployed for most smart phones. Has anyone used this website to do something similar? Any advantages or disadvantages I should know about and tell him? Maybe someone could give me a better explanation on what this really does.
I'm current a one man army. He wants the application out for most smart phones but can afford to hire more developers.

Rhomobile will start up a small webserver on your phone and then show a webview that is directed to this webserver. You are able to write all the application logic in ruby in a way you would do it if you would write a web app that is deployed on a real web server. Rhomobile uses CSS etc. to have the app look look a little bit like a native app.
If you know Ruby you will get an App fast but it will look crappy and the user experience will be crappy too. A similar framework is appcelerator titanium they will let you write the app in Java Script and then compile it into a mix of javascript, webview and native components that run on an Iphone and on an Android phone and titanium has a much nicer user experience then rhomobile.
Visit their pages and test some of the apps that they are promoting as showcase for their frameworks.

As many developers writing apps in Objective C have learned using HTML for the view and styling with CSS styling libraries is a great way to create attractive native apps. Rhomobile's Rhodes uses this approach as well. We used to let people use whatever CSS they wanted and they could choose to use IUI, JQTouch, IWebKit, WebApp.net or any other library along with Rhodes. The best external apps all seemed to use JQTouch for styling and animated transitions.
Recently we decided to ship with our own fork of JQTouch (which we made work on Android) in order to make this even easier. The recent Rhodes master branch created apps with JQTouch builtin makes such attractive apps even easier to build, as the stylesheets are included with Rhodes scaffold-generated apps.