The unconscious status causes the subject to be unable to act and suffer a large defensive penalty. Any hostile action will wake the subject, so the full penalty only lasts for a single initial attack, though it is likely that the subject will also have been rendered prone by falling unconscious.
This condition is most commonly induced by the Sleep spell. Undead are immune to this effect, but may be affected by the Symbol of Sleep society power. Constructs and elementals are generally unaffected.