hmmm... well, you can go to redoctane.com or click the pumpitup link i put up there, which has a page (press "2" when you get there, i think) that shows how the game works.
basically there is a platform in front of the screen, with pressure-sensitive panels that you dance on. the screen has arrows that show you which panel to step on at what time... they scroll up the screen and when they hit the top is when you're supposed to step on that panel! and when you step on the panel it lights up. there are many different songs you can choose, as well as different levels on the songs - the harder levels have more and/or faster footwork. i'm not sure how DDR grades things but PIU grades you at the end of a song, from A to F. the at-home versions have different modes and you have to buy a footpad, which is a Playstation attachment for DDR and a slightly different pc attachment for PIU. If you're looking to try it out, call around to some local arcades and ask if they have Dance Dance Revolution or some similar "dance game". and for heaven's sake start on the easy mode... even that was too hard for me. :)