区别：A FragmentActivity is
a subclass of Activity that
was built for the Android
Support Package.
The FragmentActivity class
adds a couple new methods to ensure compatibility with older versions of Android, but other than that, there really isn't much of a difference between the two. Just make sure you change all calls to getLoaderManager() and getFragmentManager() to getSupportLoaderManager() and getSupportFragmentManager() respectively.
使用：use Activity if
you are using android.app.Fragment;
use FragmentActivity if
you are using android.support.v4.app.Fragment.
Never attach a android.support.v4.app.Fragment to
a android.app.Activity,
as this will cause an exception to be thrown.
http://stackoverflow.com/questions/10477997/difference-between-activity-and-fragmentactivity