How does armor work? Shogun 2 Total War

I've played Shogun 2 for a while now, and something I still don't understand is armor. The unit with the most notable armor is the Naginata Samurai with 9. Compare that to a Yari Ashigaru that has 2 armor, and it seems like there's a huge difference. I can see the number value of unit armor, but I don't understand is how that factors into combat. Does armor reduce the damage done by arrows? Does armor just give an increased dodge chance?

I think these might be questions of how combat is calculated by the game engine and not just of armor, but armor is my real concern. I can immediately see the effects of increased attack or morale, but not as much by increased armor.

How does armor affect the survivability of a unit?