For people who have read Mortal Intruments: What curse did Clary give Simon?

She put the mark of Cain upon his forehead. This means that anyone who kills him will 'suffer vengeance seven times over' from the Christian god. It also says that the bearer of the curse 'will be a restless wanderer on the earth', so that may or may not have an effect of Simon. She cursed him so the vampires wouldn't kill him for being a daywalker. The effect of Jace's blood was that Simon became able to withstand sunlight.