Unconscious creatures are knocked out and helpless. Unconsciousness can result from having negative hit points (but not more than the creature's Constitution score), or from nonlethal damage in excess of current hit points.
Marius, Scholai Armsmaster Says: Never, ever leave an unconscious character underwater. Since they cannot hold their breath, they will be dead in two rounds. You can feed an unconscious character a potion as a standard action.