Some magic creatures have the supernatural ability to instantly heal damage suffered from weapons, or ignore blows altogether as though they were invulnerable. Sometimes Damage Reduction (DR) represents such instant healing. Sometimes it represents the creature's tough hide. In any case, when a character sees their attacks are failing, they may switch to a different type of attack.
A creature's DR is the amount of damage the creature ignores from normal attacks. Usually, a certain type of attack can overcome this reduction. This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons except magical ones. If a dash follows the slash, then the damage reduction is effective against any attack, except those that ignore DR.
The weakness types are not absolute, however, and powerful enough magic weapons can overcome some forms of damage reduction. +1 or better weapons bypass damage reduction with /magic, +3 or better weapons bypass /silver or /cold iron, +4 or better weapons bypass /adamantine, and +5 weapons bypass any alignment (good, evil, lawful, chaotic).
Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury, poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact. Attacks whose damage is cancelled by the target's DR do not disrupt spells.
Damage reduction only works on pure weapon-style damage. Spells, spell-like abilities, and energy attacks (including non-magical fire) ignore damage reduction, even if they are delivered by a standard attack.
If a creature enjoys damage reduction from more than one source, the reductions do not stack. Instead, the creature receives the benefit of the better damage reduction given the situation. For example, if creature has DR 5/magic and 10/cold iron, then it reduces a standard weapon by 10, a cold iron weapon's damage by 5, a magic weapon's damage by 10, or a magic cold iron weapon by nothing.