great post arkov. Wish I had read a post like this back when I first bought the game, since I was a fan of fantasy games but at the time knew nothing about the D&D specifics.
Armor Classes (AC) are somewhat of a tough topic to conquer for the new D&D or NWN player because they have so many ways to modify them.
Something Arkov left off was that you get boosts to your AC the higher your dexterity goes and for every 5 ranks in the tumble skill.
These bonus's work like this:
(someone correct me on these next statements if I'm wrong any of it)
You are completely naked (no items of any kind equiped) and your AC is 9 and your Dexterity is 10. If you increased your Dex to 12, which would give you +1 bonus to that ability, that bonus ALSO is applied to your AC giving you now an AC of 10 instead of 9. HOWEVER, this +1 bonus is a DODGE bonus, not a NATURAL bonus, even though it may look like a natural one because it boosted your AC when you were naked.
As far as the Tumble skill goes, for every 5 skill points you put into this skill you also get a +1 bonus to your AC, and again, I believe, this is a DODGE bonus, not a Natural one. So if you have 10 ranks in Tumble, you would gain a +2 bonus to your AC
Like Arkov said, dodge bonuses can stack on top of one another so you can have a dex of 20 (granting a +5 dodge) and a Tumble of 10 (granting a +2 dodge) and you get a +7 AC bonus to top of whatever other armor your and items you have.
Just to give you an example of how this would look, lets say a rogue has:
20 dex (+5 dodge)
10 tumble (+2 dodge)
wears light armor (+2 armor)
Has a cloack (+1 deflection)
has an amulet (+1 Natural)
and a base AC of 9 (natural) to start with at character creation
Giving this rogue an ac of 21. Not too shabby. Lets also not forget then that Halflings also get a dodge bonus (inherent bonus feat called 'small stature') on top of all that, when fighting anything bigger than they are (including dwarves). so a halfing rogue is damn hard to hit.
Anyway, I hope this completes the AC picture for anyone not sure about it all.