A helpless character 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 (equivalent to attacking a prone target). Ranged attacks get 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 bow or crossbow, provided he is adjacent to the target. The attacker automatically hits and scores a critical hit. (A rogue also gets his sneak attack damage bonus against a helpless foe when delivering a coup de grace.) If the defender survives, he must make a Fortitude save (DC 10 + damage dealt) or die. Delivering a coup de grace provokes attacks of opportunity.
Creatures that are immune to critical hits do not take critical damage, nor do they need to make Fortitude saves to avoid being killed by a coup de grace.
Marius, Scholai Armsmaster says: Be mindful that oozes, elementals, and incorporeal undead are immune to critical hits and thus cannot suffer a coup de grace. This is different from earlier game systems--you can coup de grace corporeal undead and constructs. If you frequently make your opponents helpless (such as with the Sleep or Hold Person spells), then you may want to equip your party with weapons that deal lots of damage on critical hits like scythes, picks, axes, and punching daggers. Even a helpless spellcaster can be dangerous if they have a spell with no components. These do not exist normally, but with a metamagic feat, a helpless wizard could still cast a Silent Teleport or silent still Charm Person.