[QUOTE=Fiberfar]Correct me if I am wrong but I think Natural Ac is the hardness of your skin ect. [/QUOTE]
Yep, the various armor boni are "explained" like this:
Natural AC Modifer:
Toughness of skin and direct dermal armoring (claws, horns, horn plates and the like). Basically only orcish/abysmal beings and barbarians should have this, but even your well-educated city sorceress can get a leathery skin with the right amulets. Question is if she'd like it.
Armor AC Modifier:
The various layers of metal that your character wears for protection. The more, the better, but at some point it will restrict your movements, thus spellcasters (who wave their hands around a lot) will get penalties to their spellcasting. Item types here are armor (duh), bracelets/gauntlets ... and helmets, iirc.
Shield AC Modifier:
Same idea as the armor one of course, only that you need a free arm for it.
Deflection AC Modifier:
In principle, this one is about various invisible magic fields surrounding the wearer/caster which either weaken or divert physical assaults. This is the most common form of magic protection and can be found on several item types, starting with cloaks and rings, but also belts. Since this type of protection doesn't hinder movement at all, it is the prime choice for mages. Also about the only kind of AC boosting that can be found as spells to cast.
Dodge AC Modifier:
Describes how nimble your character is and decides if he/she can roll with blows or sidestep them completely. First half of this you get from your DEX ability score, the second half from boots and spells. Dodge is the only AC bonus that can be raised by wearing multiple items/effects of the same type, in all other bonuses only the highest current modifier will count.
(meaning wearing a +1 deflection ring and a +4 deflection cloak -wont- give you a +5 bonus, but only a +4 one... since the +1 is ignored then.)
+ highest NaturalModifier
+ highest ArmorModifier
+ highest ShieldModifier
+ highest DeflectionModifier
+ Dexterity-based DodgeModifier
+ Item-based DodgeModifier
+ Feat-based DodgeModifier
Yet, note that all Dodge Modifiers will be zero'ed or lowered as long as your character is slowed, frozen, paralyzed, encumbered (partially), entangled... and dazed, too I think.
Side question: is there any spot in NWN where you attacks/round are shown? Being a paper&pen-player I have the attack progression tables in mind anyway... but it feels kinda lacking not to see that (highly important) data anywhere on the NWN character sheet.