Armor Class

Armor class (AC) is a number representing a creature's ability to avoid being hit in combat. An opponent's attack roll must equal or exceed the target creature's Armor Class to hit it.
Armor Class = 10 + all modifiers that apply
Modifiers typically include:
*Armor bonus
:* AC Bonus (normal armor and armor enchantments)
*Shield bonus
:* AC Bonus (normal shield and shield enchantments)
*Natural armor bonus
Modifiers below are applied against Touch attacks; the ones above do NOT.
*Deflection bonus
*Dodge bonus
*Dexterity modifier
*Size modifier
*Luck bonus
== Full explanation (Definitely) ==
Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It’s the attack roll result that an opponent needs to achieve to hit you. Your AC is equal to the following: 10 + ( armor bonus + natural armor bonus ) + shield bonus + deflection bonus + ( Dexterity modifier + dodge bonus ) + size modifier.
Note that armor limits your Dexterity bonus, so if you’re wearing armor, you might not be able to apply your whole Dexterity bonus to your AC.
Sometimes you can’t use your Dexterity bonus (if you have one). If you can’t react to a blow, you can’t use your Dexterity bonus to AC. (If you don’t have a Dexterity bonus, nothing happens.)
Other Modifiers: Many other factors modify your AC.
* Armor enhancement bonus: Enhancement effects make your armor better.
* Shield enhancement bonus: Enhancement effects make your shield better.
* Deflection bonus: Magical deflection effects ward off attacks and improve your AC
* Natural armor bonus: Natural armor improves your AC; should be stackable along with your normal BASE AC.
* Dodge bonuses: Some other AC bonuses represent actively avoiding blows. These bonuses are called dodge bonuses. Any situation that denies you your Dexterity bonus also denies you dodge bonuses. (Wearing armor, however, does not limit these bonuses the way it limits a Dexterity bonus to AC.) Unlike most sorts of bonuses, dodge bonuses stack with each other.
Apart from dodge, none of the effects stack and only the highest applies.
== Loss of Dodge and Dexterity bonuses ==
Whenever a character is flat-footed, she loses any applicable dexterity and dodge bonuses. Characters which are stunned, asleep or paralysed, or else not actively engaged in combat are considered flat-footed. Some feats, such as Uncanny Dodge, will allow a character to retain dex bonuses to armor class if the character is caught flat-footed.
NOTE: Although being flanked will render a character vulnerable to sneak attacks just as being caught flat-footed will, a character who is the target of a sneak attack is not automatically flat-footed.
== Touch attacks ==
Touch attacks: Some attacks disregard armor, including shield bonuses and natual armor. In these cases, the attacker makes a touch attack roll (either ranged or melee). When you are the target of a touch attack, your AC doesn’t include any armor bonus, shield bonus, or natural armor bonus. All other modifiers, such as your size modifier, Dexterity modifier, luck, deflection and dodge bonus (if any) apply normally.
The reason you disreguard your armor, shield, and natural armor bonuses is while these modifiers help stave off weapon blows, it is still assumed that you are sometimes "hit" by a weapon, or block with a shield for no damage, and since the goal of a touch attack is to merely touch you, these bonuses are not taken into account.
AC would be calculated at:
 10 + deflection bonus + ( Dexterity bonus + Dodge AC bonus ) + Size modifier