Android Activity Lifecycle

The Android mobile operating system has a well-defined life cycle for Activities and application components running in the OS.

Lifecycle events can happen even if the current component is not handing off control to another Activity explicitly -- the mobile OS is a very busy place and things happen constantly that you, as the developer a single application, cannot control.

For example, perhaps a call comes in to the handset. If this happens, and your Activity is running, it will be swapped out while the call Activity takes over.  What is an Alarm goes off?  What if the task killer elects to eliminate your package?

http://developer.android.com/guide/topics/fundamentals.html#lcycles