It's like a laptop mouse pad, your fingers do all the hard work of moving the arrow on your screen (joking).
Just incase you haven't seen or don't know what a magic trackpad is I've got some images.
One reason why I like the magic trackpad is that you can do many more new movements to control your computer. Such as (if your using a mac) fore fingers swipe across to move between full screen application, fore fingers swipe up will take you to mission control, and again fore fingers swipe down will take you to app Exposé. All up there are about 15 different movements that you can do with one hand to control your mac.