Basically, the rules are the same as the version in the books. There are two teams of seven players. Three are Chasers, whose job it is to take the Quaffle up the field and score through the hoops. Two are Beaters, who take the Bludgers and beat people on the other team. Then there is the Keeper, who guards the hoops and starts the play. Lastly, there is the Seeker, whose job it is to catch the Snitch and end the game.
The differences are that players don't actually fly, but run around with brooms in between their legs. The Quaffle is a volleyball, the Bludgers are dodge balls that are thrown by the Beaters, the hoops are made out of PBC pipes and hoola-hoops, and the Snitch is a person. When you are hit with a Bludger, you are "beat" and have to run back to your hoops before continuing to play. To catch the Snitch, the Seeker has to pull a tennis ball stuffed in a sock stuck in the back of the Snitch's pants. And instead of earning 150 points for catching the Snitch, the team only gets 30.
It's a full contact sport, with a lot of tackling and no protective padding. It's also replacing ultimate frisbee at most colleges.
Dan: Dude, you should play Muggle Quidditch!
Jared: Aw, man, that game was rough! I got beat five times in a row, and got tackled by the Keeper before I could score! Then, the other team's Seeker got the Snitch before we could get 30 points up.
Fred: Wait, do you play Muggle Quidditch?
Jared: Yay! It's like the cooler version of rugby!