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When to start separate process in android ?

I have 100 of images to download from server and each one size is nearly 200kb.To serve this purpose in the app,I may have to go for creating Service OR Intent service. I know that intent service is best fit in this situation because it runs on worker thread. but my only concern is, my application will also initiate other threads for different needs. so too many thread at time may leads to performance degradations. so i am in dilemma of making decision for intent service or service in separate process altogether. creating separate process might be benefitted in all the way. what do you say ?

It doesn't matter. If you remember your first multi-threading class you might be told that all threads don't work simultaneously but it is like "one runs for a while then second runs then one runs again then maybe third and so on" order is random. It doesn't affect performance because for the processor it doesn't matter if you use 2 threads or 100.
However make sure you don't wait for , say first 10, images to download first so that user don't have to wait too long for images to download o a slow connection.

Of course that Intent Service running in worker thread and Service running in Main Thread. So if you want to download images in Service you still need create new Thread so it's same work here, but Intent Service have some advantages over Service :
1.Background thread ( You no need to create Thread to download images when Service need)
2.Automatic queuing the Intents delivered to onStartCommand(), so if one Intent is being processed by onHandleIntent() on the background thread, other commands queue up waiting their turn.
3. the automatic shutdown of the IntentService when job done (in service you need to call stopService)
About create new Process it's no need. Just create one IntentService to download your images. With bigger data you can make multiple thread and download multiple part at same time but 100 images and 200kb each is really no need it.

How to create a service that runs in the background and performs async tasks in Android

I'm having great difficulty understanding when to use Service vs IntentService in Android.
I'm trying to create a Manager Class that can, download, verify and install APKs.
The process of doing this require me to spawn a service(DownloadManager) to download the file, which causes my service to be destroyed prematurely.
It also needs to run an activity to install the apk.
This download manager has no front end, I just want it to be a background process that does its thing and returns the results programmatically.
I've read into both Service and Intent Service and although the documentation clearly says that Intent Services are meant to be used when the processing should be done off the UI thread, but nearly every forum I visit says that you should not do async work inside an IntentService.
For example:
Waiting for asynchronous callback in Android's IntentService

In general, an IntentService is useful for when you have discrete tasks that you want executed one at a time off of the UI thread. The IntentService will keep track of each request in a queue, execute one request at a time - on a separate, non-UI thread - and then will shut down when the queue is empty. If a new request arrives later, it will start up again, then shut down again once the queue is empty.
The warnings about running "async" work inside of an IntentService are because once onHandleIntent exits, the IntentService thinks that item has finished processing. It has no way of knowing if you created another thread that you want it to wait for. So once it has called onHandleIntent for all outstanding requests, it's going to shut down, even if there are child threads still running.
A non-intent Service gives you control over when the service starts and stops, regardless of whether there's any work to do. Also, unless you specifically make it otherwise, everything the Service does happens on the UI thread - so if you want work done on a background thread, you need to explicitly implement that. It's also up to you to implement how the Service handles multiple incoming requests. But, the service won't shut down until you tell it to (or the OS runs out of resources).
It sounds based on your description like you probably have two choices:
If you're ok with the service processing requests one at a time, you could use an IntentService - but you'll need to make onHandleIntent wait for each request to finish. This is still happening off of the UI thread, but it does mean that if you have multiple download requests, they're not going to happen in parallel.
You could use a non-intent Service to process each download request on its own child thread, all in parallel. Then it's up to you to keep track of all the processing threads.

Asynctask vs IntentService for syncing with backend mysql server

My application does a sync process every 30 seconds.
What I do is, send some data, and accordingly get a set of data, and update/insert into my local sqlite DB. Sometimes, I updated my GUI as a result of these operations.
I read in several posts that an asynctask can do this task perfectly, and in some other posts many people discouraged the use of asynctask, and told me to implement the intentService/broadcast receiver approach.
Which one is better? and when ?

There are multiple considerations that you must weigh in order to best decide how to approach your situation. Acording to the doc:
...an activity that initiates a long-running
operation might do well to start a service for that operation, rather
than simply spawn a thread — particularly if the operation will likely
outlast the activity. Examples of this are playing music in the
background and uploading a picture taken by the camera to a web site.
Using a service guarantees that the operation will have at least
"service process" priority, regardless of what happens to the
activity.
A Service is a part of your Application that has no UI. It may be called by a UI(Activity) to be started, or may be started by any other component of your Application. When developing, you have the freedom to place it on a different thread, or even run it in a different Task or Process. This allows you to ultimately separate it from your UI. Additionally, you may start the Service to run independently (startService) or bind your activity to it (bindService) depending upon your needs. By using custom Handlers, you can set callbacks to update the UI with your progress. A Service does not necessarily end if a User changes Activities, but may be ended at ANY time by the OS.
A AsyncTask is always instantiated from the UI thread. It only allows specific callbacks, but simplifies the process of multi-threading for the purposes of relatively short transactions (as compared to dedicated separate threaded services) that are inherently tied to actions performed by an Activity. Whenever a User changes Activities, the AsyncTask is put on "pause" and may even die because there is no UI thread for your Activity any longer.
The problem with the AsyncTask is if the user goes to another Activity you can't transfer that object to the other Activity so it dies. There are tricks you can play when say the user rotates the screen or something like that, but that doesn't extend to general purpose destruction. AsyncTask can randomly die. On the other hand you can definitely communicate between Service and Activity, but it's tricky to do it right.
So, depending on your scenario, you have to take a design decision.

If you need to update the UI then AsyncTask will be a better fit, however if you need it to update in the background when the app is not running then IntentService would be better and you will need to come up with a way to update the UI from your IntentService
This is more of an opinion based question so I would not expect one definite answer. Its a per-app basis question

Why would one use AsyncTasks when you can use Services?

When you learn to develop for Android, you learn that if there's any type of process -that takes more that 5 seconds- running inside the ui thread, the system will show the infamous ANR message. That's when they introduce the evil monster... AsyncTask. At first you see this class as your savior, but then you realize it's the biggest problem in development... Handling rotation or ui events becomes so problematic, it's even painful.
What do you guys think about it... are AsyncTasks worth the trouble? Any other method that makes it less of a problem to perform long running tasks regardless of screen rotation?
I guess you'll probably say, hey why don't you just make it a Service... Yeah that's a solution, but then let's go to my real question. Why don't we just use services and stop using AsyncTasks completely... Are they useful at all?

While developing it is always good to choose the suitable component for the execution. You need to clear your requirement and according to it you can choose between a Service or an AsyncTask.
A service is an Android component that lives independently from any other components. Activities may come and go, but services can stick around, if you wish. They serve a number of roles, from managing state that multiple activities rely upon to serving as cron jobs to handling longer app widget updates.
AsyncTask is a class that arranges to do some work off the main application thread. From the implementer and user of the AsyncTask, how it performs that bit of magic is not generally important. In reality, it uses a thread pool and work queue. AsyncTask uses a Handler to help arrange for select bits of work to be done on the main application thread (e.g., notifying the user of progress updates).
For more clarification see the difference between them.

AsyncTasks and Services are not peers. Either might invoke the other, their purposes are different. I have just a few things to add to Lucifer's answer.
A key thing to remember is that if your process is in the background, hosts a Service, and is killed, that Service will be reincarnated at some point. This is not the case for an AsyncTask that may be running in a process when it is killed. This means that work being done in a Service is guaranteed to make forward progress in the future (although its not guaranteed when). If you have work that must be done, better to do it in a Service.
Calls to Service.onStart and Service.onStartCommand happen on the process' main thread. If you block this for a long time, you'll get the same "app not responding" dialog. As a result, an AsyncTask can be a great thing to use inside a Service to do the real work.
I tend to use AsyncTasks initiated from an Activity for things related to the UI. These are often things that aren't required work and whose work can be easily redone in the future. For example, this might be reading a bunch of images off disk or make a calling to a web service to see if the user has any notifications. This work can be done again, and if the UI is destroyed because the process is killed the work probably needs to be done again anyway.
If on the other hand you want to say upload a group of photos to the web as soon as possible, I'd farm this out to a Service which will use one or more AsyncTasks to do it. This is work I want to get done and want to be resumed if it gets interrupted. If you're going to use a Service, consider using an IntentService so that your Service stops when its work is done.

Android: AsyncTask vs Service

Why do I read in the answer to most questions here a lot about AsyncTask and Loaders but nothing about Services? Are Services just not known very well or are they deprecated or have some bad attributes or something? What are the differences?
(By the way, I know that there are other threads about it, but none really states clear differences that help a developer to easily decide if he is better off using the one or the other for an actual problem.)

In some cases it is possible to accomplish the same task with either an AsyncTask or a Service however usually one is better suited to a task than the other.
AsyncTasks are designed for once-off time-consuming tasks that cannot be run of the UI thread. A common example is fetching/processing data when a button is pressed.
Services are designed to be continually running in the background. In the example above of fetching data when a button is pressed, you could start a service, let it fetch the data, and then stop it, but this is inefficient. It is far faster to use an AsyncTask that will run once, return the data, and be done.
If you need to be continually doing something in the background, though, a Service is your best bet. Examples of this include playing music, continually checking for new data, etc.
Also, as Sherif already said, services do not necessarily run off of the UI thread.
For the most part, Services are for when you want to run code even when your application's Activity isn't open. AsyncTasks are designed to make executing code off of the UI thread incredibly simple.

Services are completely different: Services are not threads!
Your Activity binds to a service and the service contains some functions that when called, blocks the calling thread. Your service might be used to change temperature from Celsius to Degrees. Any activity that binds can get this service.
However AsyncTask is a Thread that does some work in the background and at the same time has the ability to report results back to the calling thread.
Just a thought: A service may have a AsyncTask object!

Service is one of the components of the Android framework, which does not require UI to execute, which mean even when the app is not actively used by the user, you can perform some operation with service. That doesn't mean service will run in a separate thread, but it runs in main thread and operation can be performed in a separate thread when needed.
Examples usages are playing music in background, syncing data with server in backgroud without user interaction etc
AsyncTask on other hand is used for UI blocking tasks to be performed on a separate thread. It is same like creating a new thread and doing the task when all the tasks of creating and maintaining the threads and send back result to main thread are taken care by the AsyncTask
Example usage are fetching data from server, CRUD operations on content resolver etc

Service and asynctasks are almost doing the same thing,almost.using service or a asynctask depends on what is your requirement is.
as a example if you want to load data to a listview from a server after hitting some button or changing screen you better go with a asynctask.it runs parallel with main ui thread (runs in background).for run asynctack activity or your app should on main UI thread.after exit from the app there is no asynctask.
But services are not like that, once you start a service it can run after you exit from the app, unless you are stop the service.like i said it depends on your requirement.if you want to keep checking data receiving or check network state continuously you better go with service.
happy coding.

In few cases, you can achieve same functionality using both. Unlike Async Task, service has it's own life cycle and inherits Context (Service is more robust than an Async Task). Service can run even if you have exited the app. If you want to do something even after app closing and also need the context variable, you will go for Service.
Example: If you want to play a music and you don't want to pause if user leaves the app, you will definitely go for Service.

Comparison of a local, in-process, base class Service✱ to an AsyncTask:
✱ (This answer does not address exported services, or any service that runs in a process different from that of the client, since the expected use cases differ substantially from those of an AsyncTask. Also, in the interest of brevity, the nature of certain specialized Service subclasses (e.g., IntentService, JobService) will be ignored here.)
Process Lifetime
A Service represents, to the OS, "an application's desire to perform a longer-running operation while not interacting with the user" [ref].
While you have a Service running, Android understands that you don't want your process to be killed. This is also true whenever you have an Activity onscreen, and it is especially true when you are running a foreground service. (When all your application components go away, Android thinks, "Oh, now is a good time to kill this app, so I can free up resources".)
Also, depending on the last return value from Service.onCreate(), Android can attempt to "revive" apps/services that were killed due to resource pressure [ref].
AsyncTasks don't do any of that. It doesn't matter how many background threads you have running, or how hard they are working: Android will not keep your app alive just because your app is using the CPU. It has to have some way of knowing that your app still has work to do; that's why Services are registered with the OS, and AsyncTasks aren't.
Multithreading
AsyncTasks are all about creating a background thread on which to do work, and then presenting the result of that work to the UI thread in a threadsafe manner.
Each new AsyncTask execution generally results in more concurrency (more threads), subject to the limitations of the AsyncTasks's thread-pool [ref].
Service methods, on the other hand, are always invoked on the UI thread [ref]. This applies to onCreate(), onStartCommand(), onDestroy(), onServiceConnected(), etc. So, in some sense, Services don't "run" in the background. Once they start up (onCreate()), they just kinda "sit" there -- until it's time to clean up, execute an onStartCommand(), etc.
In other words, adding additional Services does not result in more concurrency. Service methods are not a good place to do large amounts of work, because they run on the UI thread.
Of course, you can extend Service, add your own methods, and call them from any thread you want. But if you do that, the responsibility for thread safety lies with you -- not the framework.
If you want to add a background thread (or some other sort of worker) to your Service, you are free to do so. You could start a background thread/AsyncTask in Service.onCreate(), for example. But not all use cases require this. For example:
You may wish to keep a Service running so you can continue getting location updates in the "background" (meaning, without necessarily having any Activities onscreen).
Or, you may want to keep your app alive just so you can keep an "implicit" BroadcastReceiver registered on a long-term basis (after API 26, you can't always do this via the manifest, so you have to register at runtime instead [ref]).
Neither of these use cases require a great deal of CPU activity; they just require that the app not be killed.
As Workers
Services are not task-oriented. They are not set up to "perform a task" and "deliver a result", like AsyncTasks are. Services do not solve any thread-safety problems (notwithstanding the fact that all methods execute on a single thread). AsyncTasks, on the other hand, handle that complexity for you.
Note that AsyncTask is slated for deprecation. But that doesn't mean your should replace your AsyncTasks with Services! (If you have learned anything from this answer, that much should be clear.)
TL;DR
Services are mostly there to "exist". They are like an off-screen Activity, providing a reason for the app to stay alive, while other components take care of doing the "work". AsyncTasks do "work", but they will not, in and of themselves, keep a process alive.