You can get your detailed answer by reading The Android 3.0 Fragments API blog. One of the relevant point is:
Embedded Activities via ActivityGroup were a nice idea, but have always been difficult to deal with since Activity is designed to be an independent self-contained component instead of closely interacting with other activities. The Fragment API is a much better solution for this, and should be considered as a replacement for embedded activities.
You can also look at Is it advised to build all apps using fragments? question. Hope it helped.