The Bludger is one of the three kinds of balls used in the game of Quidditch. It is directed, both independently and through Beaters, towards players in an attempt to knock them off their broomsticks.
Back in the eleventh century, in the earliest, most rudimentary forms of the game, rocks were enchanted to rocket around and try to knock players off their brooms. They came to be known as "Blooders", the first mention of that name being in the twelfth century. Eventually they evolved into rocks carved into the shape of balls. By the fifteenth century, however, the Beaters' bats had evolved to become magically reinforced, and thus were able to crack the rocks, leaving the players to be followed by flying gravel instead. That is why, in the early sixteenth century, teams began to experiment with metal Bludgers. At first, they began to use lead instead of rock, but eventually lead was deemed too soft and prone to indentation, which would affect the Bludger's flying ability, and so lead was replaced with iron, which is still used today.
Bludgers are bewitched to rocket around the Quidditch pitch and to chase all players indiscriminately. However, a Bludger may be enchanted to follow a particular player relentlessly, though that would be against the rules of the game.
 Role in Quidditch
Two Bludgers are released in a game of Quidditch, and they spend the duration of the game flying around, trying to knock players off their broomsticks, thus considerably complicating the game. Two Beaters on each team, armed with small clubs, attempt to knock the Bludgers away from their team's players and towards the opposing team.