From FIFA Laws of the game (I know, they are not always enforced…)
A player is sent off and shown the red card if he commits any of the following seven offences:
1. is guilty of serious foul play;
2. is guilty of violent conduct;
3. spits at an opponent or any other person;
4. denies the opposing team a goal or an obvious goalscoring opportunity by deliberately handling the ball (this does not apply to a goalkeeper within his own penalty area);
The ball is played towards goal by the attacking team and a defender jumps and handles it as the attacking player moves towards the ball. The defender is sent off for denying the opposing team an obvious goalscoring opportunity.