How does Armor work exactly, RE: armor Penetration

How does armor work?

For example, if i have 100 armor total.

And someone swings a sword at me for 50 damage before armor.

After armor, how much damage do i actualy take?

I ask because certain skills have armor penetration ratings, and if that's only going to allow for 1 or 2 more damage, totaly not worth it.

Thanks for any help and clarity on this issue, much appreciated.

G.

All your armor questions are answered here