Android App Close or Exit Buttons
Android apps do not have Close or Exit buttons. A well-designed Android activity should stop when it is no longer visible on the device screen and resources are needed elsewhere.
Built-in Application Manager
Third-party Application Managers
Problems that can occur when you have many apps open
Performance degradation
What happens when you are using a child screen, navigate away from it, and then reopen the app?
What does the Android device Back button do?
References
Navigation with Back and Up
http://developer.android.com/design/patterns/navigation.html
Providing Proper Back Navigation
http://developer.android.com/training/implementing-navigation/temporal.html
Back and other hard keys: three stories
http://android-developers.blogspot.co.il/2009/12/back-and-other-hard-keys-three-stories.html
Stopping and Restarting an Activity
http://developer.android.com/training/basics/activity-lifecycle/stopping.html
Tasks and Back Stack
http://developer.android.com/guide/components/tasks-and-back-stack.html
Providing Ancestral and Temporal Navigation
http://developer.android.com/training/design-navigation/ancestral-temporal.html
Activities
http://developer.android.com/guide/components/activities.html
Pure Android
http://developer.android.com/design/patterns/pure-android.html
Recreating an Activity
http://developer.android.com/training/basics/activity-lifecycle/recreating.html
Action Bar
http://developer.android.com/guide/topics/ui/actionbar.html