Warriors are the tank. Period. Sure paladins can tank, and hunters' pets can tank... but none as efficient or as good as us warriors.
We have the best armor in the game, tied with paladins. We have a lot of abilites that have high aggro (sunder armor is great for this, slap a few on and it's difficult to pull aggro from me)
And, of course, taunt. If you are the tank and you aren't using this, you aren't doing your group any favors.
That's not to say warriors can't do anything else, we have decent dps, we can stun, aoe fear (Haven't used this much, only one time where it did any good)
If you are, or planning to be a warrior. Best advice I can give you is, get tactical mastery asap, use taunt whenever you lose aggro and switch stances according to the situation... everything else you should be able to pick up.