The Tank Role in ESO - What does a Tank do?

There are a few particular types of skills that are important to tanks, enabling them to aggro enemies, stack them in one area for easier DPS, buff and protect allies and debuff enemies. The following are the kinds of skills that you should consider using, but it's not an exhaustive list and some dungeons and trials require slightly different skills to others. 

The taunt is the most important skill that a tank has. It is essentially an attack that forces an enemy to attack them for a set period of time. When it expires the enemy will need to be taunted again, otherwise it may turn around and start attacking your team. 

Pierce Armor is the best taunt in the game, as it reduces the armor of the enemy by the greatest amount alongside the taunt effect itself. However it is also melee only, so it's a good idea to also have a backup ranged taunt, such as Inner Fire. 

It's a good idea to use an armor buffing skill that will give you the Major Resolve buff, significantly increasing your damage mitigation. The best option for this is undoubtedly the Balance skill, as using this will also convert some of your health to magicka, allowing you to sustain your other skills more easily. It is best to avoid doing this when you are under heavy pressure, but if your healer is decent then you will barely even notice the health cost of this skill. 

There are two types of crowd control that you will want to have on your tank. The first is a pull - an ability like Unrelenting Grip, which will pull smaller trash enemies to you. Most bosses are immune to this, but you can use it against smaller enemies to “stack” them, making them easier to control and allowing the damage dealers in your team to damage them more easily. If you're not a Dragonknight you can instead use the Silver Leash skill. Wardens also have the option of using Frozen Gate, but this is a bit slower. 

The second type of crowd control is a root. This will immobilize enemies for a few seconds. It is more optional, you technically don't need it but it can make it easier to control large numbers of enemies. An example of a root skill is Choking Talons. 

Generally, tanking works best when you focus on mitigating or avoiding damage and relying on your healer to top up your health. However there are situations where you won't get much healing from others - because your healer died, or you are far away from them due to a mechanic, or they simply aren't very good at healing. For these situations it doesn't hurt to have a self heal slotted. Health-scaled burst heals like Coagulating Blood or Dark Cloak are generally the best options, but if your class doesn't have something like this then you could try something like Resolving Vigor instead. 

It's generally a good idea to put Elemental Blockade on your back bar, with an infused Ice Staff with the Crusher enchantment, to support your group by reducing the armor of enemies. 

Dragonknights can make plentiful use of the Igneous Shield skill. Used correctly, this skill provides a lot of utility both for the tank, and for the group. The shield itself is obviously handy for mitigating damage against yourself and your allies, but in conjunction with passives in the Earthen Heart skill line it gets even better. The Helping Hands passive essentially generates free stamina when you use the skill, helping you to sustain block, and you also give the group the Minor Brutality buff with the Mountain's Blessing passive. At the time of writing, there is no other way to get this buff apart from each individual damage dealer using Increase Weapon Power poisons. So you are doing your team a big favor by using this, unless there are other Dragonknights in the group also using Earthen Heart skills regularly. 

Often, bosses will start channeling an attack that does a lot of damage to your team if it is not interrupted in time. If you are next to the boss then you can simply bash them to prevent this from happening, but some bosses like to move around a lot and won't just stay on the tank. For these situations, it's good to use Crushing Shock as a ranged interrupt to avoid being caught off guard.

Finally, let us consider the ultimate abilities that we can use on our tank. Generally it is a good idea to have a defensive ultimate and an offensive ultimate (the offensive ultimate is focused more on buffing the team's damage output, as our own DPS is a tank is simply not going to be very good). 

A great option for a defensive ultimate is to put Replenishing Barrier on your front bar. This protects both you and your team with a huge damage shield when you use it, which is great for tricky moments. You will also get an extra 10% magicka recovery just for having it slotted, as long as you unlocked the Magicka Aid passive. You can also use more selfish ultimates here though, such as Magma Shell or Spell Wall. 

For the offensive ultimate, typically slotted on the backbar, there are two options. Necromancer tanks can use Frozen Colossus to debuff enemies with Major Vulnerability. Otherwise, you can use the Aggressive Horn skill to grant your group Major Force, significantly increasing group DPS output.