Basically, you pay however much you want to pay for a bundle of games. Proceeds go to charity. If you pay more, you get more. But the base bundle is usually pretty awesome, and you can pay literally 1 cent if you want.
Its an awesome system, and they've raised a lot of money through it. It looks a little sketchy, but they send you the games through steam (as far as I've experienced) so its all legit.