Priest is the class you are looking for. Druids are also good healers, though not as dedicated. They are more of a mix between all the different classes (Regular form = Spellcaster, Bear form = Warrior, Cat form = Rogue). Paladins are Alliance (blech) exclusive, and are basicly warriors with healing capabilities. The Horde counterpart is Shaman, which is a pretty balanced fighter/spellcaster with healing capabilities.