Table of Contents
Elemental Vulnerability
7 damage types have Elemental vulnerabilities to them: 
Fire, 
Shock, 
Cold, 
Acid, 
Poison, 
Iron, and 
Salt.
For Fire, Shock, Cold, Acid, and Poison
Whenever a unit is hit by a damage type they're vulnerable to, if it's one of these four types, the initial damage (what goes into the Damage DRN roll) is either doubled or increased by the point value of the vulnerability; whichever adds less.
For example, say a Jotun Bondi with 
Shock Vulnerability (5) is hit with a Thunder Strike. The damage of the main part of the Thunder Strike is usually 26; since this is shock damage, however, it's increased to 31. This alone is just enough to kill them in one hit. The secondary damage of the spell is usually 2; this is also shock damage and, since 2 is less than 5, the damage is only doubled (to 4).
Within the game's code, there is actually no such thing as an Elemental Vulnerability. It is recorded as a negative Elemental Resistance to the same damage type. Therefore, adding 10 Elemental Resistance to 5 Elemental Vulnerability results in 5 Elemental Resistance.
Poison and Acid Vulnerabilities are typically rarer than the other three. In particular, Poison Vulnerability is mainly a side-effect of various items, while Acid Vulnerability is only found on the stone-skinned Trolls. The other three are applied by various buffs to Natural Protection; Barkskin for Fire, Stoneskin for Cold, and Ironskin for Shock.
For Iron
An Iron Vulnerability adds to the damage roll from iron weapons and certain iron attack spells, specifically those that proceed against you. Blade Wind and Iron Darts are iron attack spells, for instance, while the iron in arrowheads is not considered to be enough iron to trigger the vulnerability. Its only difference from the above five is that it isn't capped at doubling the base damage.
Note that iron doesn't have a damage icon; for some weapon types (where it isn't obvious), you may need to click on them and see if they are made of iron. However, not many magic weapons are made of iron.
For Salt
A Salt Vulnerability changes the damage cap on Enchanted Salt. For those without Salt Vulnerability, Enchanted Salt's damage is capped at zero after the Damage & Protection DRN rolls; thus, this trait literally is a measure of how vulnerable one is to Salt. Even Salt that doesn't do damage can Stun people, though.