ActionBarSherlock : Use Action Bar Design Pattern Across All Versions of Android
ActionBarSherlock is a library that lets you implement action bar design pattern across all versions of Android through a single API. It uses the native ActionBar implementation on Android 4.0 or later and a custom action bar implementation for previous versions which allows for action bar implementation in all versions of Android from 2.x and up.
The same functionality can also be implemented using the Android Support Library (appcompat). However, if you want to know more about the subtle difference between using either of this libraries, I will recommend you read through this stackoverflow thread that deals with it in details and also has some updated information.