The main game has you setting out a random game and playing a round. When that is complete, the players who have won most (or lost fewest) points take one of the rules cards to show their victory, giving a new game for the next round. Play continues until someone has won four rules cards.
If you've read this description, and a cross between Fluxx and Bridge doesn't appeal, that's fair enough. On The Cards tends to polarise people into those who love it and those who can't see the point, and you're likely to be in the latter camp.
If, on the other hand, you're thinking 'that sounds fascinating', you can get a copy from the publishers at Surprised Stare Games, or directly from me if you know you're going to see me and want to save the postage. Or if you want to ask questions, please feel free to leave a comment below.