Damage reduction

Absorption is an effect found on some Warrior-specific equipment that reduces damage. All sources of absorption are global; they will reduce damage no matter which part of the body is hit. There are three different sources of absorption: armor, runes, and shields. Different sources of absorption stack, but similar sources of absorption do not. To get maximum damage reduction, you only need one Rune of Absorption and one piece of damage-reducing armor. Additional runes and armor will not reduce damage further.

Sources of Absorption

 * Ascalon Armor and Knight's Armor reduce taken damage by 2. Absorption from Knight's/Ascalon armor affects most sources of damage, although there are a few skills like Giant Stomp which are not reduced by armor absorption. Knight's/Ascalon armor has the weakest armor level of the Warrior armors, so they are normally worn on a harder to hit location like the boots or hands. However, as sometimes Warriors use Stonefist Gauntlets, the boots are the most effective location. Ascalon armor does not stack with Knight's armor; you only need one of either.
 * Runes of Absorption reduce damage depending on potency. Minor Runes of Absorption reduce damage by 1, Major by 2, and Superior by 3. Runes of Absorption only reduce damage from Attacks and have no effect on spells or other sources of damage. Like all runes, Runes of Absorption do not stack with each other, even though they do not have (non-stacking) in their description.
 * Shields have several reduction modifiers, each with different requirements. They reduce damage from all sources, even those that armor absorption doesn't reduce. A shield has at most two mods.

Absorption chart
Note that the 1-10% chance shields are from before the March 02, 2006 update which increased the maximum from -3 (10% chance) to -5 (20% chance).