Fragments

Whereas an activity is part of an app, and an app can have many activities, a fragment is part of an activity.  A fragment controls one portion of your display image, where the activity is responsible for the entire display image.  But you can imagine a display broken up into pieces, maybe one piece is a scrollable list, another is a series of buttons or something, one is a text window where information appears.  Each of these is controlled a fragment, and whichever fragment is "active" that's the one whose code is presently running.