Quidditch is the most popular sport in the Wizarding World. Now, Quidditch is easy enough to understand, even if it's not too easy to play. There are 4 kinds of balls. Also, there are 7 players on each side. 3 of them are called Chasers. The Chasers throw the Quaffle which is a bright red ball about the size of a football and try and get it through one of the hoops to score a goal. Those were 3 golden poles with hoops, that reminded Harry of the little plastick sticks Muggle children blew bubbles through, except that they were 50 feet high. Anyways, back to the game. 10 points everytime the Quaffle goes through one of the hoops.
Now, there's another player on each side who's called the Keeper. The Keeper has to fly around the hoops and stop the other team from scoring. So basically, there are 3 Chasers and 1 Keeper who play with only 1 Quaffle.
There are 2 other balls which are called Bludgers. They rocket around trying to knock players off their brooms. That's why every team has 2 beaters. It's their job to protect their side from the Bludgers and try and knock them towards the other team with a small club. The Bludgers are jet-black and slightly smaller than the Quaffle.
The last member of the team is the Seeker. There is also one last ball which is the Golden Snitch and it's the most important ball of the lot. It's very hard to catch because it's so fast and difficult to see. It's the Seeker's job to catch it. Whichever Seeker catches the Snitch wins his/her team an extra 150 points, so they nearly always win. The Golden Snitch is bright gold and has little fluttering silver wings.
A Quidditch game only ends when the Snitch is caught.

Gryffindor Team
Captain: Oliver Wood
Keeper: Oliver Wood
Chaser: Katie Bell
Chaser: Angelina Johnson
Chaser: Alicia Spinnet
Beater: Fred Weasley
Beater: George Weasley
Seeker: Harry Potter

Slytherin Team
Captain: Marcus Flint
Keeper: Bletchly
Chaser: Marcus Flint
Chaser: Montague
Chaser: Warrinton
Beater: Bole
Beater: Derrick
Seeker: Terence Higgs (book 1) - Draco Malfoy (books 2 & 3)

Ravenclaw Team
Captain: Roger Davies
Seeker: Cho Chang

Hufflepuff Team
Captain: Cedric Diggory
Seeker: Cedric Diggory