Largely based on equipment, and unique equipment is mostly used for additional abilities rather then improved stats. For example heavy armor gives you a good 10% Damage Resistance, and a heavy shield adds another 5%. That being said, you can still circumvent those bonuses if you fight smart. For example, stab a dood in the eyes where he don't have armor and you can negate some of his DR from his armor. Also some general make-sense stuff; fighter types could get away with surviving getting impaled a few times, but a wizard better have a damn good reason if he can do the same. 

There isn't any hard stats per say, but at the same time there's a general guideline for the power and strength of characters based on their class, feats, abilities, equipment, etc.