Armor penetration

This term refers to the amount of damage that ignores an opponent's armor, effectively reducing the target's armor level (often abbreviated as "AL") by that percentage.

Armor penetration is more effective against higher AL but still stronger against lower AL, just like everything is stronger against lower AL.

Armor decrease
For instance, if you use an attack or skill that has 25% armor penetration against an opponent with 80 AL, that attack would give him -20 AL (20 = 25% of 80).

10% armor penetration against 100 AL lowers it by -10 AL.

Damage increase relative to damage without armor penetration:
A decrease of -40 AL equals 100% more damage. A strike that would normally cause 20 points of damage to do 40 instead.

A decrease of -20 AL equals 50% more damage. A strike that would normally cause 20 points of damage to do 30 instead.

A decrease of -10 AL equals 25% more damage. A strike that would normally cause 20 points of damage to do 25 instead.

An increase by 10 AL equals 25% less damage. A strike that would normally cause 20 points of damage to do 15 instead.

...

AL relative to armor penetration:
Inflicted damage without armor penetration (left) decreases faster than inflicted extra damage by armor penetration (right), the higher the AL is.

for more details see: Damage calculation