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A nuker is a character (or creature) that uses ranged damage spells as offense, rather than being a Tank or Healer. Their higher damage ability is usually balanced with low defense; the Mage is the prototypical nuker, though Warlocks and Hunters can also serve as nukers. The term is somewhat misleading in the context of World of Warcraft, as several other classes can perform this role without necessarily being archetypal casters; the preferred and more neutral term is simply damage per second or (playfully) pewpewer.
For tanks, threat generation is back-loaded. They start slowly, then their threat generation takes off. The nuker should wait for the tank to get aggro before attacking, and then keep the DPS low so his/her threat is low. This will keep the the mob off of him/her, thus letting the tank absorb the damage. No tank wants to run around trying to save the nuker. Low aggro or threat helps the healer, letting him/her save mana for the tank instead of wasting it on the nuker.
After a couple of minutes or when the mob is nearly dead, the nuker can go all out and bring down massive nuking to kill the mob before it runs away. This is not only appropriate, it's appreciated, saving the tank from running after the mob and keeping it from bringing its friends to the party. Besides that, it also looks dramatic and cool.
Also, see Caster.
Nuker ability comparison by class
Good: These classes are excellent at laying down massive and withering fire from afar.
- Mage (Almost exclusively damages with magic from range, has several AoE damage abilities, has more options and variety in means of magical damage than virtually any other single class)
- Hunter (Primarily does physical damage, but does so from a safe distance)
- Warlock (Damages slowly; In long fights can sometimes catch up or surpass mages and hunters)
Fair: These classes can contribute from a distance, but can probably be helping in other ways as well.
- Shaman (Mainly meant for nuking in the Elemental tree, but can be done in others, at least for a short time.)
- Priest (When not healing, a reasonable nuker, see also "Face Melt")
- Druid (Slightly more flexible than Priest, but similar applies. If skilled right (Balance) a Druid is a great nuker.)
- Death Knight (Although mainly a melee class, Death Knights are very capable nukers at relatively close range.)
Bad: These classes have no business hanging at the back, and should get stuck in.
- Warrior (Poor ranged weapon skill only. Charge!)
- Paladin (May do a degree of nuking if properly specialized and equipped for Holy Shock and other spell-damage abilities)
- Rogue (Should already be behind the target when the action starts)
Note: similar to what stated on the top of this page, this comparison is out-of-date and misleading. It is based on the ability of the classes to deal damage from a distance. In reality, any of the ten classes can possibly top the damage output in the group/raid with good gear, spec and skill.