Some monsters are vulnerable to piercing, bludgeoning, or slashing damage.
Some monsters are vulnerable to certain materials, such as alchemical silver, adamantine, or cold-forged iron. Attacks from weapons that are not made of the correct material have their damage reduced, even if the weapon has an enhancement bonus.
Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters. Such creatures’ natural weapons (but not their attacks with weapons) are treated as magic weapons for the purpose of overcoming damage reduction.