Becomes available at level 10 and grants special bonuses related to the classes
5 levels in Cleric + 5 levels in Wizard or Sorcerer = Mystic Theurge
5 levels in Ranger + 5 levels in Cleric = Druid
5 levels in Fighter + 5 levels in Wizard or Sorcerer = Swordmage
I'm sure there are others but i can't think of them right now. Also there could be a much more powerful version available at level 20 for those that are 10 and 10.