the Sneak Attack link
If you go there you can find an FAQ covering this very specific topic, and as it is open source I will post it here:
Precision damage (such as that dealt by a rogue's sneak attack ability) applies to more creatures than it did in previous editions of the game.
Some may balk at this but it can easily be imagined or explained as the rogue having found a weak point in the undead's "body" (such as a zombie's head) or even finding a crack or flaw in a construct's "body."
There is some degree of confusion as to what should separate a "critical hit" from a "precision-based attack" but in any event, in some cases they are treated differently.
Critical Hits
The following creature types (or subtypes) have immunity to critical hits (that is, they do not take any additional damage from critical hits):
Aeon (subtype): "Immunity to cold, poison, and critical hits."
Elemental (subtype): Elementals are "not subject to critical hits or flanking. Does not take additional damage from precision-based attacks, such as sneak attack."
Incorporeal (subtype): "An incorporeal creature is immune to critical hits (unless the attacks are made using a weapon with the ghost touch special weapon quality.)"
Ooze (Type): <Oozes are...> "not subject to critical hits or flanking. Does not take additional damage from precision-based attacks (such as sneak attack.)"
Protean (subtype): (50% chance to ignore, see below*)
Swarm (Type): "A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits."
Precision-Based Damage (like Sneak Attack)
The following creature types (or subtypes) do not take additional damage from precision-based attacks (such as sneak attack):
Elemental (subtype): "<An elemental...> does not take additional damage from precision-based attacks (such as sneak attack.)"
Incorporeal (subtype): "An incorporeal creature is immune to precision-based damage (such as sneak attack damage) unless the attacks are made using a weapon with the ghost touch special weapon quality."
Ooze (Type): "<An ooze is...> does not take additional damage from precision-based attacks (such as sneak attack.)"
Protean (subtype): (50% chance to ignore, see below*)
Creatures Immune to Flanking
Opponents do not gain any special flanking bonuses against the following creature types (or subtypes):
Ooze (Type): "<An ooze is...> not subject to ... flanking."
Swarm (Type): "A swarm has no clear front or back and no discernible anatomy, so it is not subject to flanking."
Elemental (subtype): "<Elementals are...> not subject to flanking."
*Special: Proteans have a special ability called "Amorphous Anatomy" which might protect them: "<Amorphous Anatomy> grants <a protean> a 50% chance to ignore additional damage caused by critical hits and sneak attacks,"