Shields in Dark Souls: PtRE provide AC is a much different way from armour, as opposed to providing differing levels of AC depending on the damage type, shields provide a blatant addition to all ACs however, shields have a damage reduction, when an attack hits the AC that a shield provides, the damage is reduced by the % stated by the shield. Shields also allow for parrying, which you can read about here and provide different parry bonuses depending on the shields type/size.