For questions about navigation within and between Android apps. Examples include navigating between activities/fragments, up/back behavior, task stacks, and Intent flags that affect navigation behavior.
Consistent navigation is an essential component of the overall user experience. Few things frustrate users more than basic navigation that behaves in inconsistent and unexpected ways.
Android 3.0 introduced significant changes to the global navigation behavior. Thoughtfully following the guidelines for Back and Up will make your app's navigation predictable and reliable for your users.
Android 2.3 and earlier relied upon the system Back button for supporting navigation within an app. With the introduction of action bars in Android 3.0, a second navigation mechanism appeared: the Up button, consisting of the app icon and a left-point caret.
See also the official documentation about this topic.