There's a lot of explaining that goes along with it.
Basically you have 3 Chasers, 2 Beaters, 1 Keeper and 1 Seeker on each team. The Chasers pass a red ball called the Quaffle. They fly down the field and try to throw it through 1 of 3 hoops. Each goal is worth 10 points. The Keeper protects the goals and tries to block the Quaffle from going on. The Beaters use these bats to hit a ball called the Bludger and they're used to knock players off their broom. The Seeker aren't normally with the group, they have their own task. There's a small golden ball called the Golden Snitch. The Seeker's job is to catch it and that's worth 150 points. The game ends when the Snitch is caught, and typically the team that catches the Snitch will win the game. However if the Chasers from the other team make enough points, the extra 150 points from the other team might not matter.