When you take damage, you may have a modifier to the amount of damage taken attached to your character, or abilities used on them. These modifiers are known as Defence, Resistance, Weakness, Vulnerability and Immunity.
Resistance and Weakness are calculated first, then Defence and Vulnerability. If a creature with Resistance to a damage type gains a Weakness to the same damage type, the damage is calculated normally.
Your Defence rating is a value that is subtracted from the amount of damage you take of its matching type. 4 Fire Defence would reduce 10 points of fire damage taken to 6 points of fire damage taken, and 5 Slashing Defence would reduce 5 Slashing Damage to 0.
Resistance to a damage type halves incoming damage of that type.
Weakness to a damage type doubles incoming damage of that type.
Your Vulnerability rating is a value that adds a flat amount of damage to any source of damage you take of its matching type. 5 Slashing Vulnerability would add 5 points of damage to a hit that dealt slashing damage. Vulnerability counters Defence, and vice versa. If a creature has 4 Fire Defence and is cursed to have 10 Fire Vulnerability, they then have a total of 6 Fire Vulnerability.
Immunity to a damage type or condition causes the damage type or condition to have no effect.