A Quidditch match is played by two teams made up of seven players. There are three Chasers, two Beaters, one Keeper, and one Seeker. Chasers keep possession of the quaffle and try to score a point by throwing it through one of three hoops on the opposing team's side. They can pass the ball to each other but only one is allowed in the scoring area at any time.
The Keeper protects the hoops from an oncoming quaffle by flying in front of them in the scoring area. Beaters protect their teammates by hitting bludgers away from them and usually toward the opposing team. They are not allowed to aim at referees, crowds, or the Keeper, unless the quaffle happens to be in the scoring area.
The Seeker's job may sound easy but it is probably the most important and difficult position of the game. They're in charge of capturing the golden snitch, which happens to be about the size of a walnut and tries its best to keep away from players. Once the Seeker successfully catches the snitch, they earn their team 150 points and end the game. Most times, whichever team catches the snitch wins.
The Quidditch world cup is held every four years, with top of the line brooms, somewhere where there is the littlest chance of muggles coming across it. If muggles do, by chance, come close to the pitch, they suddenly remember about urgent appointments and run off in another direction. Very convenient.