For the Horde, the same classes are used, though obviously linked to different races.
Blood Elf: Mage, spams Fireball, will sometimes use Polymorph.
Undead: Rogue, uses Deadly Poison and drops poison clouds.
Troll: Hunter, haven't really seen him use any abilities. Has no pet.
Orc: Warrior, uses Mortal Strike and Whirlwind.
Tauren: Shaman, has been seen using Chain Lightning and a spell called Hex of Mending (effect unknown, we interrupted it).
Night Elf: Hunter
Same abilities as well.