Damage Reduction (Su): The creature 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 magic weapon or a creature with its own damage reduction can sometimes damage the creature normally, as noted below.
The entry indicates the amount of damage ignored (usually 5 to 25 points) and the type of weapon that negates the ability. For example, the werewolf's entry reads "damage reduction 15/silver": Each time a foe hits a werewolf with a weapon, the damage dealt by the attack is reduced by 15 points (to a minimum of 0). However, a silver weapon deals full damage.
Any weapon more powerful than the type listed in the note also negates the ability. (For details, see Table 3-13: Damage Reduction Rankings, page 74 in the Dungeon Master's Guide. )
For example, the werewolf (damage reduction 15/silver) takes normal damage from weapons with +1 or better magical bonuses, but not from nonmagical weapons made from material other than silver, and not from keen weapons or weapons with other magical properties.
For purposes of harming other creatures with damage reduction, a creature's natural weapons count as the type that ignore its own innate damage reduction. However, damage reduction from spells, such as stoneskin, does not confer this ability. The amount of damage reduction is irrelevant. For example, a Large air elemental (damage reduction 10/+1) deals full damage to a werewolf, as if the elemental's attack were with a +1 weapon.
---pg. 9-10, MM