Dodgeball is basically a mini-war that usually happens in a gymnasium or some other enclosed arena thingy. Two teams take each side of the area, normally played on Basketball courts here in America, and balls are placed down the center line of the court. The coach/official/whatever blows the whistle, and the folks go running for the balls, hoping to grab as many for ammunition against the other team.
From then on, it is an all out war between the two teams (the more people the better, IMO), and play can get exciting pretty fast. You're out if you get hit with the ball, however, hits in the head are considered a penalty, and the person who threw the ball is out. Same goes for people catching the ball, the person who threw it is out. The ball can also be used as a deflector for the other balls, but those rules depend on where the game is being played.