It's pretty, though, ain't it?
Well, it works on the principle of planetary rollers - think along the lines of a rear diff on a car - the screw turns, little planetary rollers (smaller diameter screws with the same thread depth and pitch) roll in opposite direction (just like balls in a ball screw) around the circumference of the screw. The little planetary rollers also roll INSIDE a nut wich, also and predictably, has the same thread depth and pitch. So looking from the side at the whole concoction you in effect end up with a normal bearing with the shaft (screw) in the centre, balls or rollers (planetary roller screws) around the shaft and race (nut) on the outside.
I'll see if I can dig up a few drawings to make things easier to picture.