ianhanniballake is right. You can get all the functionality of Activity from FragmentActivity. In fact, FragmentActivity has more functionality.
Using FragmentActivity you can easily build tab and swap format. For each tab you can use different Fragment (Fragments are reusable). So for any FragmentActivity you can reuse the same Fragment.
Still you can use Activity for single pages like list down something and edit element of the list in next page.
Also remember to 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 an android.app.Activity, as this will cause an exception to be thrown.