[video]
You get a set of colored pellets with paint inside them and shoot them at people--kind of like you would with a normal bee-bee gun. But when the pellets hit something they break open and the paint splatters all over the place.
If you're playing a game with teams the point if for one team to beat the other team. If you get hit in a vital place, you're technically "dead" for the rest of the game. When everyone on the other team is "dead" your team wins. If it's one-on-one, you have to hit the other person more times than they hit you.