Threat is an easy concept to understand, but difficult to actually play around. Put simply, "threat" is how big a threat a given mob sees you as. Each action you take that affects that target will increase the amount of threat you have generated toward it. If an enemy feels more threatened by you than its current target, it will switch and start attacking you instead!
Please note that the terms "aggro", "threat", and "hate" are used somewhat interchangeably; however this tends to confuse things somewhat. It is more precise to say that whoever is being attacked by a mob has "aggro," but whoever is highest on the hate list has the highest "threat." These two are not always the same player, particularly for bosses in higher end instances.
When you take an action against an enemy, it will generate a certain amount of threat toward it. All threat generated is added to the mob's hate list. As a general rule, the player at the top of the hate list is the mob's target, but there are some exceptions. (See sticky aggro below.)
Note that hate lists are not shared, and every creature has their own unique list. Slamming one mob with a Pyroblast won't make his buddies even blink unless it's the first attack, and even then only because they just realized you exist. However, nearly all beneficial effects - buffing, healing or sometimes just gaining mana or HP will cause you to gain threat with all the mobs that you're currently engaged with. Especially in many instances tanks will have to maintain at least enough aggro on multiple mobs to prevent them from going after healers, even without players using AE attacks.
Also known as "building aggro," nearly all actions you take will increase your threat total on some or all targets. There are 4 ways to create threat on the hate list, which include;
The simplest and most common is simply doing damage to a mob. Each point of damage you deal (before threat modifiers, such as Blessing of Salvation) is worth one point of threat. Some attacks cause additional threat beyond their damage values. For example, Heroic Strike causes an additional 196 threat, and Sunder Armor causes 301 bonus threat while dealing no damage at all. In general, effects that debuff the mob or prevent them from acting (such as Counterspell) tend to generate significant amounts of threat in addition to their base damage.
The next most common way to gain threat is via healing, which includes effects that restore or give mana or even rage. By default each point of healing is worth half a point of threat. (Each point of mana restored is also worth half a point of threat and each point of Rage is worth five points of threat.) This threat is applied to all mobs that you or your target are currently engaged with; however, this threat is also split between all of the mobs. For example, if Alice heals Bob for 1000, Alice will generate 500 threat. If they're engaged with 5 mobs, each mob gets 100 threat towards Alice. This tends to mitigate the difficulty of keeping mobs off of your healers, which would otherwise be nearly impossible for tanks to achieve.
Buffing tends to cause very small amounts of threat, but it noticeable when buffs occur on the pull. When someone has more aggro than the tank on all the targets, it can end badly. Like healing, this threat is split between all mobs.
Finally, proximity aggro: Mobs will generate an extremely small amount of threat towards the first person they see. In addition, certain mobs are social, and will assist their friends if they are attacked. If any social monster is pulled near his friends, they will also generate aggro towards you as if you had generated proximity aggro - even if the mobs are not hostile by default.
Combat pulses are exclusive to boss mobs, especially in raids. Every few seconds during which a boss is engaged it will generate one point of threat on every player in the zone, which puts every player into combat. This is used to prevent players from remaining out of combat and rezing dead players or other tricks.
There are three conditions where a mob will switch targets.
In order to prevent threat "ping-ponging" between multiple targets, the current aggro target is sticky. You have to do more than just be at the top of the hate list in order to become the new aggro target.
If you are in melee range, you need to be at 110% of the aggro target's threat in order to unstick him from the tank. If you are outside of melee range you need 130% of the aggro target's threat in order to unstick him from the tank.
As a consequence of this, however, if anyone does exceed the aggro limits and become the new tank they are likely to kill themselves and others. Other players who were above the tank but not to the point of ripping aggro will be killed before the boss returns to the tank, and if the boss has AE or frontal-arc atacks it gets even worse.
Warning: Some raid bosses will periodically 'lose' their aggro target, usually as a effect of a special attack; a good example is Magtheridon. Whenever he performs his Quake attack he will reacquire a tank afterwards based on the hate list as if he had no tank before, so if you exceed the tank's threat at all you will end up as a smear on the floor.
Some classes have a 'snap'-aggro ability, or a taunt. Taunts come in two varieties - true taunts, which increase your aggro to that of the current aggro's target permanently and force the target to attack you for a short duration, and Mocking Blow-style taunts which simply force the target to attack you for the duration without effecting the hate list. Once the effect expires the target will return to its previous hate list (including the sticky threat - it won't go after a higher-threat target unless they've exceeded the aggro threshold as per normal).
Certain effects can force a mob to switch targets because the player is no longer a valid target. Blessing of Protection makes a player immune to physical attacks, so mobs will pick the next valid target and make them the new aggro target. Other immunity effects such as Ice Block and Divine Shield will generate the same effect. Additionally, if the player is effected by most kinds of crowd control that break on damage the mobs will pick a different target - these effects include Gouge and Polymorph.
Many abilities exist that will modify your threat and threat generation. These include;
Abilities that cause a fixed amount of threat work more or less exactly as they're described; you attack with that ability, and if it connects you deal X amount of additional threat. This additional threat is modified by your global threat modifiers, rather than simply being tacked on.
Some abilities have an innate threat modifier, where that ability will always cause X% more threat. The most common example of this type of ability is Searing Pain, which causes double its damage in threat, but other examples exist.
Abilities that reduce your threat on one or more mob tend to be nearly completely useless for a variety of reasons... chief among them that they don't scale. For one, the opportunity cost is usually high; in the case of Feint, you're spending 20 Energy to reduce your threat by the equivalent of ~1000 damage, or less than a single crit, which is fine in blues but when you're non-crit autoattacks exceed the amount reduced by Feint it's a losing battle. To make a bad problem even worse, the amount of threat these attacks reduce is itself reduced by your threat modifier. For example, Rogues innately have 29% threat reduction, and Feint reduces threat by 1050. However, after the modifier they're only losing 745 threat; this still works out to Feint removing the equivalent of 1050 points of damage, but it further undercuts their scaling.
Abilities that reduce your total threat by a percentage are much more useful. The most common are the full drops - Vanish or Feign Death, but even the partial drops such as a Warlock's Soulshatter are extremely useful. Abilities that fully drop you from a mob's hate list can be used to avoid wipes and also to prevent certain effects - you can sometimes interrupt a mob's special abilities by vanishing off their hate list before they complete the cast.
Taunt and related abilities will increase your threat to that of the mob's current aggro target, as well as automatically ripping threat without needing 110% of the aggro target's threat. Note that while certain spells, such as Mocking Blow will force the target to attack you for a certain amount of time, but will not modify your threat level.
Finally, many abilities and talents will modify the amount of threat you generate with all attacks or certain types of attacks. The most common modifier is Blessing of Salvation, but many classes can reduce or increase their threat generation via talents such as Shadow Affinity or Defiance.
Note that all of percent modifiers stack multiplicatively, not additively; a Warrior with Defiance (115%) and in Defensive Stance (130%) generates 1.495x normal threat, not 1.45x. Similarly, a a Warrior in Berserker Stance (80%) with 5/5 Imp. Berserker Stance (90%), Blessing of Salvation (70%) and Subtlety to Cloak (98%) generates .494x normal threat, not .38x. As a practical matter, this greatly increases the effect of stacking threat increases and reduces the effect of stacking threat reduction.
See also tanking
The tank's job is twofold; first, to ensure that the mob(s) are hitting him, and second to ensure that he survives the mob(s) hitting him. This article references the first; for a more detailed look at the second and a more generalized view of tanking, see tank.
When tanking, you need to build aggro to hold the mob on you. You are, however, never going to be doing more damage than your DPS classes are unless something is dreadfully, dreadfully wrong. While pure damage will comprise a portion of your threat, the main ways in which you will stay ahead of the DPS on threat is due to two factors; global threat modifiers, and innate threat abilities.
Warriors and Druids both have an innate 130% threat bonus while in Defensive Stance and Bear Form respectively, whereas Paladins have 160% threat for all Holy damage with Righteous Fury active. With talents, Warriors and Druids have an additional 15% threat (for a total of 1.495) and Paladins can receive 190% threat to all Holy damage. Many of the Warrior's and Druid's attacks also cause hefty amounts of innate threat, from Heroic Strike to Lacerate.
In fights with more than one CC'd mob you will have to generate sufficient threat on all mobs for the adds to at the very least stay on you and not your healers; at best, you can generate sufficient threat that your DPS can use some of their AE abilities for extra damage. Warrior's and Druid's tend to have middling AE threat, whereas Paladins specialize in it. It is heavily recommended that as the tank you select one target as the burn target, which all DPS will focus on and then move on to other targets. This makes maintaining aggro control over the encounter much simpler, as well as leading to faster and easier clears (beyond even aggro considerations, killing mobs quicker and individually makes life easier for you and your healer(s)).
See also: healing
Be very careful of actions you take on the pull; throwing a buff or a heal right as the tank pulls will almost always lead to the mobs running right past him to turn you into a bloody paste. After the pull, responsibility for maintaining aggro on the mobs mostly falls to the tank, but there are several steps you can take to make his job easier.
Remember that any heal where the heal shows up as credited to the target and not to you in the combat log (including Earth Shield, Prayer of Mending and the final tick of Lifebloom) will generate aggro as if the tank had done the heal himself, which will greatly simplify his (and your) AE threat.
Beware of healing whenever adds spawn, especially in large groups; it's the same as a pull, but often times the tank will have to acquire the mobs as you sometimes cannot stop healing. Still, it's a good time to switch to lower-aggro heals rather than a huge heal that's going to send every one of the little buggers straight at you. With that said, any overhealing is ignored for threat generation; launching a crit heal for 8,000 when all they needed was 500 HP is wasteful, but it won't make the mobs any more annoyed with you than if you'd just healed him for the 500.
And remember that threat reduction may be just as important to you as it is to any DPS. Wisdom is great, but if there's a lot of AE pulls you may want to consider picking up Salvation from a friendly Paladin instead.
See also DPS
As a DPS class, threat control is one of your primary concerns. Fortunately, all DPS classes have ways to reduce their threat, be it through passive reduction or simply aggro reducers or wipes. Unfortunately, you'll find that's often not enough.
Unless you're in solid armor and absolutely outgear the instance, pulling aggro usually leads to your death and often the deaths of others. Don't do it. Even if the tank is generating little to no threat, try not to pull aggro. If you have to stop doing damage, stop. You're more than welcome to yell at him to generate more threat, but do not pull aggro.
Aggro reducers are your friend. Blessing of Salvation should be your god.
Some mobs have unusual hate structures that don't conform to the normal rules. Some of the more common examples of these mob archetypes include;
The best example of a constant hate resetting mob is the Lava Annihilator, which resets its hate list completely approximately once every second. As a result, it tends to go flying off in random directions unless under the effect of a Mocking Blow-style effect. Some mobs periodically enter this mode; both Battleguard Sartura and Bonechewer Blade Fury's will periodically enter a Whirlwind mode and begin chain-resetting their hate lists.
Some mobs may pick Focus Targets and then follow that player around until selecting a new Focus target. However, they do still maintain normal threat lists; the Focus Target is just treated as having a Mocking Blow-style effect up on the mob or boss until they select a new target, and their normal hate lists only come into play briefly (right before switching targets) or if their Focus target dies or becomes ineligible via an immunity effect. Thaladred the Darkener, Supremus and Buru the Gorger all share this type of effect.
Some mobs, especially spellcasters, may spend the entire fight chain-casting attacks on random targets. As with the Focus Targets, though, this is slightly misleading - often they will attack their highest-threat target once or twice between casts, which can be devastating if a lower-armored target gets hit by their melee attacks. Good examples of this type include Shade of Aran and High Astromancer Solarian.