A character’s Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on her. It’s the attack roll result that an opponent needs to achieve to hit the character through her armor. A character’s AC is equal to the following: 5 + armor bonus
When a character’s armor reaches 0 Hit Points, it no longer offers protection or a bonus to Armor Class. Excess damage from an attack is not applied to the character’s vitality points when armor reaches 0 Hit Points. Example: if a character’s armor has 10 Hit Points remaining and takes 12 points of damage from an attack, she doesn’t take the 2 points of excess damage leftover from the attack.
A touch attack is an attack roll that meets or exceeds the character’s AC without any armor bonus, but not her normal AC. If the character is wearing armor, a successful touch attack damages her armor, unless she makes a successful defense roll.
Some creatures have an AC bonus from natural armor, which usually consists of scales, fur, or layers of thick muscle. Natural armor does not have its own Hit Points and thus touch attacks do not apply. Rather, any attack roll below the creature’s AC does no damage whatsoever.