Helpless is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent's mercy.
A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target get a +4 bonus. Ranged attacks gets no special bonus against helpless targets. Rogues can sneak attack helpless targets.
As a full-round action, an enemy can use a melee weapon to deliver a coup de grace to a helpless foe. An enemy can also use a ranged weapon, provided he is adjacent to the target. The attacker automatically hits and scores a critical hit. A rogue also gets her sneak attack damage bonus against a helpless foe when delivering a coup de grace. Delivering a coup de grace provokes attacks of opportunity.