|This article needs the druid and death knight sections written.|
This article will discuss tanking, as well as providing helpful (and class-specific) tips for tanks.
A tank is the character whose job it is to absorb the damage dealt by the mobs and keep aggro off the party's or raid's other members. Tanks need to be able to manage threat on one or more mob and survive doing so.
Warriors of all types can tank, but those who specialize in doing so spec Protection. Paladins who spec Protection and Druids who spec Feral are also strong tanks. In a pinch, Hunter pets and a Warlock's Voidwalkers can tank easier mobs (such as world quest mobs), but they will not last long in instances.
Tanking is divided into two aspects; the first is generating enough threat so that the mob focuses on you, and the second is surviving the damage intake once the mob focuses on you.
Generating threat as a tank does not depend upon your damage output; your DPS will be doing far, far more damage than you are, and yet you have to retain the mob's attention regardless. The solution is passive threat modifiers. A Rogue with Blessing of Salvation will generate .49 points of threat for every point of damage he deals; conversely, a Warrior in Defensive Stance with Defiance will generate 1.495 points of threat for every point of damage he deals. Even if you're doing a third of the damage the Rogue is (and without any bonus-threat abilities), you'd be generating as much threat as he is. See the main threat article and threat values for more details on high-threat abilities.
Of course, once you have the mob's attention you need to continue living or all that threat will be worthless. Tanks focus on improving their mitigation, which includes stats like armor and shield block value, and their avoidance, which includes events like dodge and parry. Additionally, all tanks will want to reach a minimum of 5.6% crit reduction through defense and talents to prevent mobs from landing critical hits on them.
Tanks also have abilities that can be used to reduce their incoming damage; Warriors can use Shield Block to force the next one or two attacks to be blocked, thus reducing their damage, and Paladins can use Holy Shield to do the same for their next eight blocks. Druids can cast Barkskin before entering combat to reduce their damage intake as well.
Additionally, certain fights may require the stacking of resistance gear in order to mitigate or avoid some harmful spells.
Tanking as a Warrior in 5-mans will test your ability to manage threat on multiple mobs. Unlike Druids or Paladins, you don't have an easy way to get aggro on multiple mobs; your best tool, Thunder Clap, has a very high resist rate and doesn't scale, as well as having a cooldown. Thus you need to develop the skills to hold aggro on multiple mobs. While there is not set rotation that will work in every situation, there are some general guidlines to successfully tanking multiple mobs as a warrior.
Use thunderclap on every third cool-down (4.5 seconds) as well as at the beginning of the pull to establish some threat on all the targets. Use shield slam if you have enough rage that you will both be able to use your thunder clap the next time it is available, and continue to use devastate between the two. Use devastate on targets until you have maxed sunder applications on each target. Cleave should only be used if you are above 50 rage and you have a solid rage intake. It is a high rage ability and the rage to threat rate for the ability is fairly low. Use a macro to switch targets and apply devastates, or become very proficient at tab targetting or mouse-over targetting the mobs you are trying to hold.
The talent Improved Thunderclap is of great help for multiple mob tanking and should be taken if possible in the build of any warrior who intends to frequently tank instances. Shield spikes are the most effective shield enhancement for building aggro, and combined with gear that raises shield block rating substantially increase threat generation in multi-mob situations.
When pulling, establish the pull by shooting or thowing at a mob other than the initial kill target, as that will help with establishing threat on a mob you need to hold. Likewise, if there is a hunter in the party, he can misdirect a second mob onto you as well, giving more time to establish threat on whatever number of mobs remain.
Where it makes sense, picking a low health first mob as the kill target will allow your job to be made easier more quickly as your dps focus-fires on that one target. As it is a low health target, it is not necessary to build up much threat on that target and it may even be acceptable to let that mob go to the rest of the party in some situations.
Especially against single targets, you'll want to use a threat rotation to maximize your TPS. Typical threat rotations include Shield Slam, Revenge, Devastate x2 or Sunder, Revenge, Sunderx2 if you're not specced protection. For more details on what the best rotation is for you, see threat rotation.
The Paladin tanking mechanic is completely different from that of Druids or Warriors. Two things set the Paladin apart: the ability to generate aggro off the bat (frontloading aggro) and the high amount of reflective threat.
The ability to frontload aggro refers to the use of mana rather than rage. A rage bar begins empty and must be filled by taking damage or dealing it, whereas a mana bar starts as full and depletes as abilities are used. It's important for paladins to take advantage of this immediate threat generation because, while they get mana back from being healed, it's possible to become mana starved during longer fights, especially if you're not taking raid-level damage. Because of this a paladin almost always wants to start out with a high-damage ability like judging Seal of Righteousness rather than judging a non-damage seal like Seal of the Crusader.
The high amount of reflective threat allows paladins to tank an extremely high number of mobs, limited only by the ability of the paladin to stay alive during the encounter. Note that Consecration will also help with the threat generation on multiple mobs but because of the other paladin abilities, it's really not necessary to spam it. Getting multiple mobs to hit you using one consecrate is often good enough because once they're hitting you the combined damage from Holy Shield, Blessing of Sanctuary and Retribution Aura is often enough to keep their attention. Periodically dropping another consecrate is fine but spamming is usually a waste of good mana.
On trash, simply using Seal and periodic judgements of Righteousness is generally good enough as the fights usually don't last long enough for Crusader to make a difference.
For Alliance Paladins, Seal of Vengeance is generally a sub-par tanking seal to use. It does more damage than Righteousness but it is also very spiky. Steady threat generation and maintenance is always better than more, but unreliable, threat. For instance, if Seal of Vengeance hasn't hit for a while or you're out of stacks at the same time that your mage gets an unlucky crit string it's more likely that the mob is going to peel off of you than if you had been applying steady SoR hits. Also, using SoR allows you to generate 'snap' aggro without wasting taunt to return a mob to you that you can't get out of SoV.
For Horde Paladins, Seal of Blood can be a good tanking seal not only for the additional damage you put out but also because the additional damage you take means more healing and more mana returned. However, if either you or your healer is undergeared it can be more trouble and make the run much more difficult than it's worth. Use it carefully.