Many people play tanks by taunting the boss and holding block for the rest of the battle, reapplying taunts as necessary. Add self-heals and buffs to season.

Personally I think that's stupid, like healers that do absolutely nothing except heal. I always output some damage no matter what role I'm in.

On the other hand, abilities don't trigger the Ulti buff, you can't block-cast to gain Ulti, meaning you have to drop block and weave a light attack off. Apparently one-hit KOs are magnetically drawn to split second light attack weaves or something, judging by the number of people complaining how this will be the death of them.

Also check ESO Tanking Guide

A tanks job is the keep the boss from ripping through a group like wet tissue paper. Generally, a tank tries to hold aggro off the rest of its group and keep the boss where the group wants it to be. Say a boss has a huge frontal cone attack that does high damage. A tank can turn the boss away from the rest of the group (catch aggro before the attack and make the boss face away), thus keeping the rest of the group from getting wrecked. Or if a boss is easier to down in a certain area of the room (such as far away from where adds spawn), it helps to have a tank pull them into that area by attacking and pulling aggro, then leading the boss away. Tanks do have to have aggro though (which can be difficult in eso), so attacks with "taunt" effects (an effect that makes the boss focus on you) are used, such as puncture in the one hand and shield line. A tank will usually have a much bigger health pool and much more damage mitigation than a healer or dps so they can take the blows that might easily kill a more squishy party member and a good tank blocks and uses damage shields to keep themselves alive. So yes, a tanks job is damage control but also control of the boss.

Having a decent tank can also help the healer better manage their resources. A tank should ideally be able to mitigate a lot of damage through armor and shields while still being able to take some unavoidable damage by having a high health pool. If the tank is just getting wailed on without dodging, shielding, or blocking anything, they wont survive very long, even with a high health pool, and will probably cause the healer to run out of resources fast trying to heal through the bosses attacks.

Think of the tank as the guy holding down the boss while the dps wail on him. He has to be able to take the punches from the boss and is expected to do damage as well, but his primary objective is to keep him from punching anyone else in the face.

Edit: To clarify, this is a super simplified view of tanking. There is a decent beginners guide on the eso subreddit.

http://www.reddit.com/r/elderscrollsonline/comments/22qcs9/tanking_101_a_guide_for_new_and_beginner_level/