FRAGMENTS

Just as an app may contain many activites, an activity may contain several fragments.

Fragments aren't quite as easy to understand as activities, and the code is not obvious at all.

Fragments are portions of an activity screen.  you can divide the screen up into fragments, and each fragment would have its own layout.  Fragments can be further divided into other fragments.

What makes fragments cool is that your program can, at runtime, decide which should appear on your screen.