Different weapons do different kinds of damage. These in turn require different kinds of resistance to mitigate. Also, you can increase the amount of a particular kind of damage a weapon does by increasing your bonus to that damage type, using either Artifacts or Elite Tech.
This damage type occurs often in weapons that contain damage over time such as Acid Blaster or Death Cloud. A few rare examples deal only direct damage, Corrosive Teeth and Viper Fangs being good examples.
Some weapons have a DoT (Damage over Time) property. Often, their initial damage is low; the DoT is the killer. Examples of stacking DoT weapons are Acid Blaster, Skeletor Lightning, and Plasma Gun. This is not an actual damage type in and of itself; DoT can deal any of the above three damage types.