Actually Damage Reduction (DR) is one of the harder to find entries tucked away in the back of the core rule book in the appendix: special abilities section (or glossary in the online PRD).
1)
Some magic creatures have the supernatural ability to instantly heal damage from weapons or ignore blows altogether as though they were invulnerable.
The numerical part of a creature's damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). 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 that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.
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 that deal no damage because of the target's damage reduction do not disrupt spells.
Spells, spell-like abilities, and energy attacks (even nonmagical fire) ignore damage reduction.
Sometimes damage reduction represents instant healing. Sometimes it represents the creature's tough hide or body. In either case, other characters can see that conventional attacks won't work.
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
Overcoming DR: Damage reduction may be overcome by special materials, magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment.
Ammunition fired from a projectile weapon with an enhancement bonus of +1 or higher is treated as a magic weapon for the purpose of overcoming damage reduction. Similarly, ammunition fired from a projectile weapon with an alignment gains the alignment of that projectile weapon (in addition to any alignment it may already have).
Weapons with an enhancement bonus of +3 or greater can ignore some types of damage reduction, regardless of their actual material or alignment. The following table shows what type of enhancement bonus is needed to overcome some common types of damage reduction.
Damage Reduction (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 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.
Some monsters are vulnerable to piercing, bludgeoning, or slashing damage. Others are vulnerable to certain materials, such as adamantine, alchemical silver, or cold-forged iron. Attacks from weapons that are not of the correct type or made of the correct material have their damage reduced, although a high enhancement bonus can overcome some forms of damage reduction.
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.
A few very powerful monsters 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 good-, evil-, chaotically, or lawfully 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. 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 matched the subtype(s) of the creature.
When a damage reduction entry has a dash (—) after the slash, no weapon negates the damage reduction.
A few creatures are harmed by more than one kind of weapon. A weapon that inflicts damage of either type overcomes this damage reduction.
A few other creatures require combinations of different types of attacks to overcome their damage reduction, and a weapon must be both types to overcome this type of damage reduction. A weapon that is only one type is still subject to damage reduction.
2) No. (Except abilities like a paladin's smite evil and a light archon's rays, they are very few).
3) But an epic (+6) weapon gets close. a +5 weapon bypasses aligned, adamantium, cold-iron and silver DRs. An epic weapon bypasses epic mickey... I mean DR.
4) Don't be overwhelmed... get even. No wait. I think that was the wrong quote. Anyway, feel free to ask specific questions in these forums. There are nice people here that can't wait to rip out your heart...I mean answer your questions. In a kindly way.
Nope. A weapon with a +6 enhancement bonus (not effective enhancement bonus for pricing purposes). They tend to be artefacts.
When you say +6 weapon... you mean a weapon with a Total of +6 worth of enchantments and/or enhancements right?
No. He means a weapon with a +6 Enhnacement bonus. No, you cannot legally make one. You have to find one. It's an Epic (capital E)weapon.
Here's an example:
http://www.d20pfsrd.com/magic-items/artifacts/major-artifacts/axe-of-the-dw arvish-lords
If you're new to PF I'd suggest staying away from Artifacts until you've got a high degree of system mastery.
When you say +6 weapon... you mean a weapon with a Total of +6 worth of enchantments and/or enhancements right?
The weapon has to be a +6, not priced as a +6.
Example:
A +3(gives +3 to attack and damage) is priced at 18000gp
A +1 keen shocking weapon is priced as a +3, but the enhance on it is only a +1.
There are a few ways to bypass all DR. If a paladin smites an evil he creature he ignores any DR that creature possesses. Summoned lantern archons(Summon Monster 3) have 2 ray attacks that bypass all DR. There are likely more but those are the only ones I can think of off the top of my head.
