Consider a simple “tank & spank” boss fight (as a mage, you need do nothing but stand still and do damage, throughout the fight from start to finish).
You have two damaging spells: Fireball and Scorch; and one additional ability, Combustion.
Fireball and Scorch both do damage to the enemy. They differ in three ways.
Fireball takes 3 seconds to cast. Scorch takes 1.5 seconds to cast. Fireball also takes more mana to cast than Scorch.
Fireball is more DPS (it does more damage per second); Scorch is more DPM (it does more damage per point of mana expended).
Assume that you will not run out of mana. You only need to maximize DPS. Absent other differences, Fireball would be the clear choice, and your DPS rotation (sequence of spells used) would simply be: Fireball, Fireball, Fireball, ….
The third difference between Scorch and Fireball is that each time you hit a target with Scorch, that enemy is afflicted with a “debuff” (a detrimental condition) called Fire Vulnerability. Fire Vulnerability increases by 3% the damage the target takes from fire attacks. (Obviously, this includes your Scorch and Fireball spells.)
Fire Vulnerability can be applied more than once; in fact, it can be applied up to five times. An enemy afflicted with 5 instances (“stacks”) of Fire Vulnerability will take 15% (5 × 3%; the effects combine additively, not multiplicatively) more damage from all fire attacks.
