A creature with this special quality (extraordinary or supernatural) ignores damage from most weapons and natural attacks. Wounds heal immediately, or the weapon bounces off harmlessly (in either case, the opponent knows the attack was ineffective). The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities. A certain kind of weapon can sometimes damage the creature normally, as noted below.
The entry indicates the amount of damage ignored (usually 5 to 15 points) and the type of weapon that negates the ability. For example, the werewolf's entry reads "damage reduction 10/silver": Each time a foe hits a werewolf with a weapon, the damage dealt by that attack is reduced by 10 points (to a minimum of 0). However, a silvered weapon deals full damage.
Some monsters are vulnerable to piercing, bludgeoning, or slashing damage. For example, the plague spewer has damage reduction 10/slashing. When hit with bludgeoning or piercing weapons, the damage dealt by each attack is reduced by 5 points, but slashing weapons deal full 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. Examples: the war troll's damage reduction 5/adamantine, the young redcap's damage reduction 5/cold iron, and the splinterwaif's damage reduction 5/silver.
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. For example, the ssvaklor has damage reduction 10/magic and can strike as a magic weapon for the purpose of overcoming damage reduction.
Some monsters are vulnerable to chaotic-, evil-, good-, or lawful-aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. For example, many tanar'ri demons and baatezu devils have damage reduction 10/good, while the prismatic golem has damage reduction 10/evil. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature. An arrow demon, for instance, has the chaotic and evil subtypes, and thus can overcome damage reduction as if its weapons and natural weapons were chaotic-aligned and evil-aligned.
When a damage reduction entry has a dash (--) after the slash, no weapon overcomes the damage reduction.
A few creatures are harmed by more than one kind of weapon. The bonedrinker, for example, has damage reduction 5/silver or good. Either kind of weapon -- silver or good -- overcomes its damage reduction.
A few other creatures require combinations of different types of attacks to overcome their damage reduction. The sorrowsworn demon has damage reduction 10/cold iron and good, meaning that a weapon must be made of cold-forged iron and be good-aligned to overcome the demon's damage reduction. A shadesteel golem has damage reduction 10/adamantine and magic, meaning that only adamantine weapons with at least a +1 enhancement bonus deal full damage to it. A weapon that falls into one category but not the other is of no help in overcoming the creature's damage reduction -- a magic sword or a nonmagic mace is no better at harming a lich than any other weapon.