Resistances are a type of defense against spells. They are the magical counterpart to mitigation, according to the tooltip, as they absorb a flat percentage of incoming damage. It appears they also affect the chance of typed spells being fully resisted (please confirm).
There are 7 types of resistances, sometimes described in 3 different categories by resistance-increasing spells.
- Heat damage
- Cold damage
- Magic damage
- Mental (mind) damage
- Divine (spiritual/godly) damage
- Poison damage
- Disease damage
The tooltip on the Persona window states "You will absorb X% of the (resist type) damage from a level Y opponent", Y being your current level.
Increasing absorbtion% takes more resistance points as absorbtion% gets higher, however, the survivability provided by absorbtion% increases as well.
It appears that the inherent resistances are determined by race, increase upon leveling, and become more and more disparate according to race; a 70th level Erudite will have more mental and magic resistance than a 70th level Ratonga when armorless, who will have him beat on noxious resistance.
Whether a particular amount of wisdom is better than the same amount of stamina, or what a particular amount of + category or + specific resistance is worth on an item is debatable and flexible. The more health you have, the more resistance will help you.