Armor penetration

This term refers to the amount of damage that ignores the protection afforded by an opponent's armor, effectively reducing the target's Armor Level (often abbreviated as "AL"). It is generally represented as a percentage of total damage. Guild Wars has a fairly complicated formula for calculating damage, but attacks offering even a seemingly small amount of armor penetration can make a significant difference.

An AL decrease of 40 results in twice the normal damage. Therefore, if you use an attack that has 25% armor penetration against an opponent with 80AL, that attack would give him an effective 60AL. With the decrease of -20 AL, a strike that would normally do 20 point of damage would actually do 30.