Anyone who isn't ready to be addicted, back away.
Alliance: Human, Dwarf, Night Elf, Gnome, and Dreanei (i think thats how it's spelled :P).
Horde: Orc, Undead, Tauren, Troll, and Blood Elf.
There are also classes that determine how you play the game, like what abilities or spells you will obtain throughout the game.
The Rogue class for example can sneak around unseen and ambush targets from behind.
The Mage class is fragile and weak, but can cast mighty spells to kill their enemies from far away.
The newest class the Death Knight, are undead warriors, rised from the dead by the Lich King to do his bidding. However you have to have a level 55 character to make one. The level cap is currently 80 and is increasing with every expansion.
There is so much that i haven't explained and won't explain. Why? Because I want you (if you don't already have it) to buy the game. Go ahead. I promise you'll like it. And if you don't believe me, try the 14 day trial. See you in Azeroth!