Marks are one of the key elements in the game which encourage tactical approach and increase the number of possible combos. If used correctly, marks can change the tide of battle by giving you a tremendous advantage from the start, or even winning you that battle at the very last moment. Either way, if given the chance, any player, novice or expert, should take advantage of marks at any chance they can.
All the heroes can interact with Marks: put them on enemies, trigger them or take them off.

There are class Marks and faction Marks.
Some heroes can put their class Mark when they attack an enemy.
Some heroes can put a Mark of their faction by means of some of their skills.
How it works?
Once placed, Marks will be active until the marked hero is attacked by a particular enemy hero or until the Mark is removed by an ally.

Every faction has:

2 heroes that put Marks on enemies.
3-4 heroes that trigger these Marks.
1-2 heroes that remove enemy Marks from allied heroes.

Class Mark can be triggered by heroes of a particular class.
Faction Marks imply interaction of at least two heroes of the same faction.

Kobold Guard can put the Kobolds faction Mark by using his skill 'Enraging Strike' against the enemy hero. Subsequently, Spear Kobold’s skill 'Sudden Strike' inflicts critical damage on heroes with the Kobolds faction Mark.
When a mark is places on an opponent, it is important to remember that the very next attack against that character will either break or trigger the mark.
A character branded with the Melee Damager Class Mark must be attacked by a melee character for the mark to be triggered, if it’s attacked by another character then the mark will break and achieve nothing.