From FIFA.com
A player, substitute or substituted 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)
5. denies an obvious goalscoring opportunity to an opponent moving
towards the player’s goal by an offence punishable by a free kick
or a penalty kick
6. uses offensive or insulting or abusive language and/or gestures
7. receives a second caution in the same match
Page 39:
http://www.fifa.com/mm/document/affederation/federation/laws_of_the_game_0708_10565.pdf