I wrote this along with another poster at OYT. It has resided on my EzBoard since before the hack but now, to avoid losing it, I repost it here:
-DR is a means of resisting physical damage. Damage from weapon blows to a creature with DR is always reduced by a certain amount, unless certain criteria are met. The most basic DR is simply presented as a number. For example, DR 10. If no other factors are given, then simply subtract the number from any weapon damage the creature takes. For example, a creature with DR 10 struck by a longsword for 13 damage would only actually take 3 damage. No DR can reduce an instance of damage below 0. -Magic weapons can overcome DR. Each +1 of the weapon's enhancement bonus allows it to ingore up to 5 points of DR. So if the longsword in our previous example had been a +2 longsowrd, the creature would have taken the full 13 damage. If it was a +1 longsword, then the creature would have taken, 8 damage.-Some creatures have even stronger DR, and the effectiveness of magic weapons against them is reduced. For some creatures, such as Lycanthropes, Fey, strongly-aligned Outsiders, etc. This represents an even greater resistance to earthly or mundane materials. For some creatures, such as golems, it represents object-like hardness. A few creatures are vulnerable only to a particular type of weapon damage, such as slashing of bludgeoning. Lastly, some DR is unbeatable (the number is usually low), and represents extreme physical hardiness and conditioning.-If a creature's DR has a listed material, then the efficacy of magical weapons is halved, unless the weapon is made from the proper material. For example, a Satyr has DR 5/cold iron. This means that it takes a +2 weapon to get past his 5 points of DR, or a +1 cold iron weapon. Adamantine always requires a +2 enhancement bonus to penetrate each 5 points of DR, and DR X/- is not overcomable by any weapons, magic or mundane.-Outsiders with two alignment subtypes have two weaknesses in their DR, one for each of their alignment leanings. For example, a Trumpet Archon has the [lawful] and the [good] subtypes, and is thus equally vulnerable to silvered weapons and weapons made from volcanic lead. It's DR 15/ silver or volcanic lead would be beaten completely by a +3 silvered greatsword or by a +3 volcanic lead greatswrod.-Creatures that have a damage type listed after the slash have a special resistance to other types of weapon damage. For example, skeletons have DR 5/bludgeoning, because they are not greatly affected by being stabbed or cut. If the DR shows only a weapon type, as per the skeleton example, no magic is required to penetrate their DR. Any bludgeoning weapon will overcome it. However, you still may overcome 5 points of DR per +2 of enhancement bonus if your weapon is not of the appropriate type.-Some few cretures may have DR that is extremely difficult to beat because they combine effects. For example, a Rakshasa has DR 15/britghtsteel and peircing. Without a piercing, brightsteel weapon, you need +2 of weapon enhancement to overcome 5 points of the Rakshasa's DR; a weapon that simply doees piercing damage or one that is made of brightsteel will not do. A +3 brightsteel spear will beat his DR entirely.-Brightsteel is a special material that is deposited only in places where one or more of the elemental planes come together. As such it is elementally conductive, and anyone carrying or using a brightsteel weapon suffers a -1 to saves against any spell or effect that has an elemental (earth, fire, air, water) desciptor. Brightsteel has the unusal alchemical property of defeating the DR of some evil creatures.-Volcanic lead is a lead crystal that has been deposited n many, many layers, all folded onto each other to create a special crystaline lattice that happens to defeat the DR of some good outsiders. Unfortunately, it is extremely heavy, weighing 5 times as much as comparable steel weapons. This weight imposes a -1 penalty on all attack rolls made with such weapons.