A character may be resistant or vulnerable towards a damage type, or straight up immune, depending on species, feats, equipment and other sources.
Immunity
A creature that possesses complete immunity to a specified damage type, renders them impervious to any harm it may cause. They take no damage at all from attacks or effects of that nature.
Resistance
A creature that has developed a resilience against a specified damage type, allows them to endure it with greater fortitude. They take only half the usual damage from attacks or effects associated with that particular type, rounded down.
Susceptibility
A creature that is particularly susceptible to a specified damage type, makes them more prone to its negative effects. They suffer an additional half of the normal damage from attacks or effects associated with that type, rounded up.
Vulnerability
A creature that is highly vulnerable to a specified damage type, makes them especially sensitive to its harmful effects. They suffer double the usual damage from attacks or effects associated with that particular type, making it a significant weakness to be wary of.