How armor penetration works

[Орда]

Can someone explain how armor penetration vs armor works. I mean for example if i have 8 armor pen and enemy have 10 armor it means i do 2 less dmg or its calculated somehow different.
Sry for my english not my native.

[greenbrog]

armour - reduce the # of a physical damage attack hit by the armour #
armour pen reduces armour by the armour pen #

ie you hit a 50 armour target for 75 physical damage and have 25 armour pen you do
75-(50-25)= 50 physical damage

armour and elemental resistance are not the same mechanics though
elemental resistance is a %
elemental pen reduces elemental resistance by the %

ie you hit a target with 100 fire damage, they have 75% fire resistance, and you have 25% magic pen; you do
100x(.75-.25)=50 fire damage

[Орда]

Thank you!
I just have 1 of this scroll that give me 2 armor penetration and was wondering how lucky i was. Seems like raw stats is better.