Eases with an overshoot either at the beginning (easeIn), the end (easeOut), or both (easeInOut). Back is a convenience class that congregates the 3 types of Back eases (BackIn, BackOut, and BackInOut) as static properties so that they can be referenced using the standard synatax, like Back.easeIn, Back.easeOut, and Back.easeInOut.
You can configure the amount of overshoot using the config() method, like TweenLite.to(obj, 1, {x:100, ease:Back.easeOut.config(3)});