Informally, fragments are UI building blocks and are analogous to activities. They can be treated as subactivities and multiple fragments can be placed in a single activity. Fragments can represent screens within an application and are typically associated with a particular task such as a map fragment. Fragments can also exist within fragments to allow module portions of activities to be easily reused. Google formally defines a fragment as a piece of an application's user interface that can be placed in an activity (http://developer.android.com/reference/android/app/Fragment.html). Like activities, fragments have their own life cycles complete with similar callback events to use when implementing custom fragments. Fragments can also be placed on a stack for activity-like navigation through an app, complete with back button support.

Definitions and introductions aside, fragments are one of the most useful concepts to master in Android and they will make your life much easier when...