There are certain abilities that might allow for “DoT” or damage over time. These abilities are usually limited in the class skills but there are items in the game that allow for a wider range as well as a stronger DoT.

Classes that allow for DoT:
>Thieves is able to make an opponent bleed through several skills and they would lose health over time or poison their opponents through poisoning their weapons then striking someone with them
>Barbarians can also cause someone to bleed through zerk though to a lesser extent
>Clerics (priests) are able to cast a spell that inflicts damage over time by making their enemies bleed
>Mages (conjurer mages i think) are able to poison enemies through spells
>finally Druids are able to indirectly cause their opponents to catch on fire through use of the spell “flame”

Common types of DoT:
Poison
Bleed
Ice Damage (frostbite)
Fire Damage (opponent catching on fire)

and many more