Return of Reckoning

Let's suppose I have 20% armor penetration and I am going up against someone that has 20% dmg reduction from armor.
Will the extra damage I have be 20% of 20% so 4% more damage? Or, will it negate their 20% so it's as if they don't have any damage reduction?

This is an honest thought and I'm not trying to come off as whiner or anything, but just I have trouble understanding this game. I have been playing this game for about two weeks now and while I really enjoy playing this game, I don't quite understand why there are dps roles in this game. Since my t...