A player or substitute is sent off if they commit any of these:
serious foul play
violent conduct
spitting at an opponent or any other person
denying the opposing team a goal or an obvious goal-scoring opportunity by deliberately handling the ball (this does not apply to a goalkeeper within his own penalty area)
denying an obvious goal-scoring opportunity to an opponent moving towards the player's goal by an offense punishable by a free kick or a penalty kick
using offensive, insulting or abusive language and/or gestures