currentTarget is the object on which you called .addEventListener.
target is the object that originally dispatched the event.
target will only be different from currentTarget if the event is "bubbling." In other words, only from a DisplayObject on the display list that happens to be a child (or grandchild, or further) of the object to which you added the listener.
Let's say you add MouseEvent.CLICK to foo_mc, which contains bar_mc, which contains goober_mc like this:
so if you click goober_mc, you'll get currentTarget = foo_mc (because that's where you added the listener) and target = goober_mc because that's the clip you clicked.