Armor Class

Armor Class is a number that determines how hard you are to hit with an attack. A medium size creature has a base armor class of 10, which is then further modified by their dexterity modifier and also any armor that they are wearing. There are specific stacking rules with armor class bonuses.
 * A dodge bonus stacks with all the bonuses including other dodge bonuses.  (Your dexterity modifier is considered a dodge bonus)
 * A Natural armor modifier that represents creatures hide. You can have only one natural armor bonus so if you are wearing an amulet of natural armor +4 and have a natural modifier of +2 only the +4 is applied not both to give +2.
 * An Armor bonus, which varies dependant on the type of armor worn.
 * A Shield bonus, which varies depending on the shield used. Only one shield can be used at once so two shields only give the best modifier not both.
 * A Cover bonus if you are hiding behind some cover or concealed by smoke.

Some typical armor class numbers to give an example of the ranges expected.
 * 8 A clumsy person
 * 10 Average human
 * 12 Average human in leather armor or average human with a large shield
 * 17 Average human with chainmail and a shield
 * 18 Average elf with chainmail and a large shield
 * 20 Average human with Full Plate and a Large Shield