So preview version of Android API level extends just Activity
for all level, I remember from froyo and Gingerbread very well, so in these version of API level No Action bar was associated with it, or more or less ActionBar were optional and were mostly implemented with third party libraries. from jellybean, 4.2.X above ActionBar was becoming standard in Android. At this point in time Activity now extends ActionBarActivity
instead of the usual 'Activity' and after a while it become 'AppCompat'. So basically the ActionBar support was one of the major difference, as previous API level will not be able to process 'AppCompat', obviously there are some other differnce , but ActionBar is part of the major Differnce. while all API level can still handle the plain usual Activity