As of Patch 4. Armor penetration sometimes abbreviated ArP or called armor pen is a value appearing on many talents and abilities that indicates the percentage of armor that will be ignored by an attack. ArP is unique in that the more ArP you have, the greater the increase in damage becomes. Armor penetration is sometimes confused with spell penetration , even though they affect different things. Until recently, this stat has not received much attention from Blizzard, however it was seen in the talent Serrated Blades since patch 1.
Armor penetration - Wowpedia - Your wiki guide to the World of Warcraft
Armor penetration abbreviated armor pen or ArP is a value indicating the percentage of armor that is ignored by physical attacks. In Patch 4. Percentage-based armor penetration is still present in a few passive and active abilities. Armor penetration is a rating system that affects a percentage of armor ignored.
Ok, now the question is, how does this effect hunters specifically. It would seem to me that a survival hunter is still better off stacking Agility, AP, Haste before worrying about Armor Pent. The above is so wrong. The term huntard comes to mind.