Bludgers are heavy, black balls used in Quidditch. These balls fly around on their own, attempting to knock players off their brooms. The Beaters have to control the bludgers by hitting them with their bat.
The Bludger is a jet-black ball, slightly smaller than a Quaffle, which Beaters hit with a bat at opposing players during a game of Quidditch to prevent them from scoring or catching the Snitch. There are two Bludgers in a game.
During a Gryffindor versus Slytherin match, a rogue Bludger focuses only on Harry, repeatedly trying to knock him off his broom. Fred and George Weasley, the Gryffindor Beaters, believe it may have been tampered with.