Ex or Su.
A creature with this special quality 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 non-magical 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 1-/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 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 that weapon has an enhancement bonus. Examples: the werewolf's damage reduction 10/silver, the iron golem's damage reduction 15/adamantine, and the nymph's damage reduction 1-/cold iron.
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 creature's natural weapons (but not their attacks with weapons) are treated as magic weapon for the purpose of overcoming damage reduction. For example, the gargoyle has damage reduction 10/magic and can strike as a magic weapon for the purpose of overcoming damage reduction.
A few very powerful monsters, such as the solar and the tarasque, are vulnerable only to epic weapons; that is, magic weapons with at least a +6 enhancement bonus. Such creatures' natural weapons are also treated as epic weapons 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 many celestials have 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. A bearded devil, for instance, has the evil and lawful subtypes, and thus can overcome damage reduction as if its weapons and natural weapons were evil-aligned and lawful-aligned.
When a damage reduction has a dash (-) after the slash, no weapon negates the damage reduction.
A few creatures are harmed by more than one kind of weapon. The baatu demon, for example, has damage reduction 10/cold iron or good. Either kind of weapon, cold iron or good, over comes this damage reduction.
A few other creatures require combinations of different types of attacks to overcome their damage reduction. The ghaele eladrin has damage reduction 10/evil and cold iron, meaning that a weapon must be made of cold-forged iron and be evil aligned in order to overcome the ghaele's damage reduction. A lich has damage reduction 15/bludgeoning and magic, meaning that only bludgeoning 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 damage reduction, a magic sword or a non-magical mace is no better at harming a lich than any other weapon.
Based on the entry in the Monster Manual 3.5 Page 307