Natural weapons are weapons that are physically a part of a creature. A creature making a melee attack with a natural weapon is considered armed and does not provoke attacks of opportunity. Likewise, it threatens any space it can reach.
Creatures do not receive additional attacks from a high base attack bonus when using natural weapons. The number of attacks a creature can make with its natural weapons depends on the type of the attack -- generally, a creature can make one bite attack, one attack per claw or tentacle, one gore attack, one sting attack, or one slam attack (although Large creatures with arms or armlike limbs can make a slam attack with each arm). Refer to the individual monster descriptions.
Unless otherwise noted, a natural weapon threatens a critical hit on a natural attack roll of 20.
When a creature has more than one natural weapon, one of them (or sometimes a pair or set of them) is the primary weapon. All the creature's remaining natural weapons are secondary.
The primary weapon is given in the creature's Attack entry, and the primary weapon or weapons is given first in the creature's Full Attack entry. A creature's primary natural weapon is its most effective natural attack, usually by virtue of the creature's physiology, training, or innate talent with the weapon. An attack with a primary natural weapon uses the creature's full attack bonus. Attacks with secondary natural weapons are less effective and are made with a -5 penalty on the attack roll, no matter how many there are. (Creatures with the Multiattack feat take only a -2 penalty on secondary attacks.) This penalty applies even when the creature makes a single attack with the secondary weapon as part of the attack action or as an attack of opportunity.
Natural weapons have types just as other weapons do. The most common are summarized below.
Bite: The creature attacks with its mouth, dealing piercing, slashing, and bludgeoning damage.
Claw or Talon: The creature rips with a sharp appendage, dealing piercing and slashing damage.
Gore: The creature spears the opponent with an antler, horn, or similar appendage, dealing piercing damage.
Slap or Slam: The creature batters opponents with an appendage, dealing bludgeoning damage.
Sting: The creature stabs with a stinger, dealing piercing damage. Sting attacks usually deal damage from poison in addition to hit point damage.
Tentacle: The creature flails at opponents with a powerful tentacle, dealing bludgeoning (and sometimes slashing) damage.