The overwhelmingly most common way to be hurt in Norrath is by physical attack (i.e. some monster hits you). This puts a premium on defence against the physical. The usual expression of this is an over-emphasis on increasing Armor Class (AC). Resistance to the various types of non-physical attacks is therefore much underrated.
There are five non-physical attack forms, namely Poison, Magic, Disease, Heat and Cold. Each character has a resistance to each of them. The higher your resistance to an attack form, the less likely it is to do damage to you. Even if it does affect you, high resistances may reduce the amount of damage inflicted.
Resistances do not increase with level, but there are plenty of magic items that will increase them. Unfortunately most are specific to just one or two attack forms.
Not all characters start off with the same resistance abilities. There are both racial and professional variations. Religious affiliation does not seem to make any difference, which is curious. It would have been logical for say, Solusek Ro worshippers to have modifications to Heat and Cold resistances, but they do not.
Natural Racial Variations:
Barbarians live in the far icy north, and have thus become somewhat inured to low temperatures. They receive a +10 bonus to their Cold resistance.
Dwarves generally are of a hardy constitution. They receive a +5 bonus to their Poison resistance.
Halflings are of a particularly hardy constitution. They receive a +5 bonus to both their Poison and their Disease resistance.
Erudites emphasise the mind rather than the body. The attuning of Erudites to the mystical gives us a healthy +5 bonus to Magic resistance (probably the most important of the five). However, we suffer a �5 penalty on Disease resistance, due to our corresponding neglect of the physical.
Iksar are of a reptilian ancestry and are therefore poikilothermic. As a result, they gain a +5 bonus to their Heat resistance, but suffer a �10 penalty to their Cold resistance.
Trolls prefer wet, muddy, rotting places. For all their strength and size, they suffer a whopping �20 penalty to Heat resistance. Note to those involved in duels with Trolls: use fire-based attacks!
Natural Professional Variations:
Rogues get a +8 bonus to their Poison resistance. Presumably they build this up through being exposed to multitudes of nasty substances in the course of their apprenticeships.
Paladins get a +8 bonus to Disease resistance. Purity in spirit spills over into purity of the body.
Shadow Knights get a +4 bonus to both Poison and Disease. Practitioners of this profession gain great familiarity with these fell conditions.
Rangers spend all their time in the outdoors, and become accustomed to the extremes of nature. They get a +4 bonus to both Heat and Cold resistances.
Beast Lords receive a +4 bonus to both Disease and Cold resistances, for no very good reason (except possibly play balance).