When to use a Service or AsyncTask or Handler?

My rule of thumb is that an AsyncTask is for when I want to do something tied to single Activity and a Service is for when I want to do something that will carry on after the Activity which started it is in the background.

So if I want to do a small bit of background processing in the Activity without tying up the UI I'll use an AsyncTask. I'll then use the default Handler from that Activity to pass messages back to ensure updates happen on the main thread. Processing the updates on the main thread has two benefits: UI updates happen correctly and you don't have to worry so much about synchronisation problems.

If for example, I wanted to do a download which might take a while I'd use a Service. So if I went to another Activity in my application or another application entirely my Service could keep running and keep downloading the file so it would be ready when I returned to my application. In this case I'd probably use a Status Bar Notification once the download was complete, so the user could choose to return to my application whenever was convenient for them.

What you'll find if you use an AsyncTask for a long-running process it may continue after you've navigated away from the Activity but:

• If the Activity is in the background when your processing is complete you may have problems when you try to update the UI with the results etc.
• A background Activity is far more likely to be killed by Android when it needs memory than a Service.

Use Service when you've got something that has to be running in the background for extended periods of time. It's not bound to any activity. The canonical example is a music player.
AsyncTask is great when some stuff has to be done in background while in the current activity. E.g. downloading, searching for text inside a file, etc.
Personally I use Handlers only to post changes to the UI thread. E.g. you do some computations in a background thread and post the result via handler.

The bottom line: in most cases, AsyncTask is what you need.