If a rogue can catch an opponent when he is unable to defend himself effectively from her attack, she can strike a vital spot for extra damage.
The rogue’s attack deals extra damage anytime her target would be denied a Dexterity bonus to AC (whether the target actually has a Dexterity bonus or not), or when the rogue flanks her target. This extra damage is 1d6 at 1st level, and increases by 1d6 every 2 rogue levels thereafter. Ranged attacks can count as sneak attacks only if the target is within 30 feet. This additional damage is precision damage and is not multiplied on a critical hit.
With a weapon that deals nonlethal damage (such as a sap, unarmed strike, or whip), a rogue can make a sneak attack that deals nonlethal damage instead of lethal damage. She cannot use a weapon that deals lethal damage to deal nonlethal damage in a sneak attack—not even with the usual –4 penalty.
The rogue must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot. A rogue cannot sneak attack while striking a creature with total concealment.