Resist heat/cold potions and spells give temporary resistance to heat or cold. All other resistance items give permanent resistance. Two permanent resistances are not cumulative, and two temporary resistances are cumulative only in duration. Fire and cold do 1/3 damage if you have single resistance, 1/9 if you have double. Acid does 1/2 damage if you have any armor to corrode, 1/3 if you have resistance but no armor, and 1/4 if you have resistance and armor. Lightning does 1/3 damage if you have resistance. There is no resistance against poison gas.