A tank is someone who Taunts, has high enough Resistances (cap is 33k), can self Sustain, has Self-Healing, Buffs, Debuffs, and Control over the Fight.

Taunting - Abilities that say "taunts the enemy to attack you" in the description. Every tank NEEDS at least one taunt. The taunt should be refreshed before the timer runs out or else the enemy may turn around and kill a DPS or healer.

Resistances - Resistances are acquired through passives (such as Heavy Armor), you do not need to be at cap for all content and thus do not need to build heavily into resistances. The resistances will come if you've built a standard tank.

Sustain - Tanks should be able to avoid running out of magicka and stamina (leading to their deaths) without relying on a healer in most circumstances. If you're using the dungeon finder, you cannot expect healers to have any sustain for you. If you're running with a group, you can count on sustain from the healer but the healer should not have to build into sustain for you (ex. using Symphony of Blades) as that reduces overall effectiveness of the group by taking away a potential buff and/or debuff.

Self-Healing - Not everything is possible to self-heal out of but a tank should not be entirely at the mercy of a healer. Having a max health based heal is critical, especially when you end up matched with a not so good healer.

Buffs - Tanks are responsible for buffing the group just like healers are, though while healers are more buff-focused, tanks are more debuff-focused. A standard tank will bring buffs including but not limited to: Major Resolve, Major Slayer or Powerful Assault, and a source of crit (Major Force or Lucent Echoes).

Debuffs - A standard tank will bring debuffs including but not limited to: Major Vulnerability, Crusher Enchantment, and Major & Minor Breach.

Fight Control - I will elaborate more in Basic Player Skillsets, but a tank should be able to effectively move enemies, keep them grouped together, and avoid moving more than necessary.

Speed - Additionally, good tank is able to lead the charge without leaving group members behind. Groupmates should not be ahead of you, waiting for you to catch up, but you should also not pull before people have time to prebuff.

Generally, it's good to factor in one skill from each of the following categories. You can create a skill format (ex. self heal in slot 3, taunt in slot 2) for every tank build and fill out the format accordingly.
My build format:
[Chain] [Melee Taunt] [Self Heal] [Buff] [Buff] [Defensive Ult]
[Flex] [Range Taunt] [Major Resolve] [Flex] [Frost Blockade / AoE Pull] [Offensive Ult]

Please note that I am aware that more skills exist within the following categories but I cannot recommend them.

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Trash-Focused:

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Chain - An ability that pulls small enemies towards you. There are two categories, but you should at least have one single target chain. Note that these will taunt but you cannot safely use them as your main taunt as you cannot refresh taunts, only apply them.

• Single Target Chains
  Silver Leash - Stamina - Fighter's Guild - Slows
  Leashing Soul - Magicka - Scribed - Sustain/Ultimate/Major Maim/Etc.
  Unrelenting Grip - Magicka - Dragonknight (Ardent Flame) - minor amount of damage + major expedition
  Frozen Device - Magicka - Warden (Winter's Embrace) - delayed and buggy pull (places on the ground instead of connecting to an enemy) but very useful for some circumstances such as Blackrose Prison
  
  
• Area of Effect Chains
  Void Bash - 2 piece sword and board set, requires Power Slam to use. Large range. Usually set cooldown completed before needing to use again, but not always the case. Applies Major Maim to enemies. Note that Void Bash works very poorly in Moongrave Fane on the senches in particular, but all other content should be fine.
  Leashing Burst - Scribing skill, centered on yourself and pulls after 2 seconds (need to learn timing on casting it as you're coming in, before trash aggros on you, thus before ranged enemies run away). Relatively small but only takes up 1 skill slot and customizable buffs/debuffs. Pixel stacks enemies.
  Rune of Displacement - Arcanist (Herald of the Tomb). Medium range, centered on an enemy. Pixel stacks enemies. Best of both worlds between Void Bash and Leashing Burst but requires a "useless" skill line.

Immobilize & Slows - Slows are used before Area of Effect Chains to keep enemies from running away before and after the chain activates. Immobilizes are used directly after using an Area of Effect Chain to keep ranged enemies in place. You do not need both but there's no problem with having both.

• Immobilizes
  Choking Talons - Dragonknight (Draconic Power)
  Blazing Spear - Templar (Aedric Spear)
  Gripping Shards - Warden (Winter's Embrace) - Also will apply AoE Brittle if you're ice staff tanking.
  Runeguard of Still Waters - Arcanist (Soldier of Apocrypha)
  Binding Contingency - Scribing - Customizable but requires an extra input to proc the skill (I cast the skill and then immediately bash)
  
  
• Slows
  Razor Caltrops - Alliance War, also applies Major Breach
  Frost Blockade - Requires enemies to be chilled, also inflicts minor breach
  Hunter's Snare (Leashing Burst)
  Hunter's Snare (Contingency)
  
  
  Stuns and Fears - Stuns and Fears will not usually work because they have the same crowd control cooldown as being chained does (the only chain that will thus allow you to immediately fear is Rush of Agony, which is not worth using unless you're werewolf tanking). That said, some groups have really bad damage and if that is your experience, you may want to follow up the cooldown from the chain with a fear. Be careful not to stun or fear something BEFORE you chain it, though...
  https://eso-skillbook.com/skill/agony-totem Agony Totem
  https://eso-skillbook.com/skill/mass-hysteria Mass Hysteria
  

AoE Debuffs

• Major Breach
  Razor Caltrops - Alliance War, also applies a slow
  https://eso-skillbook.com/skill/unnerving-boneyard Boneyard - Necromancer (Grave Lord) - Also applies Minor Vulnerability

Expedition
Charging Maneuver ("Rapids") - Alliance War - Gives you and your group Major and Minor Expedition. Nyoom!

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Boss-Focused:

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Under Construction!

Taunts - You only really need one taunt (a ranged one, if you're only using one) but if you don't have Puncture then you need to source Major and Minor breach a different way.

• Range Taunts
  Frost Clench
  Inner Rage
  https://eso-skillbook.com/skill/runic-sunder Runic Sunder
  
• Melee Taunts
  Puncture
  https://eso-skillbook.com/skill/focused-charge Focused Charge and Its Morphs
  

Self-Heals

Defensive Abilities

• Major Resolve
• Block Mitigation
  Bracing Anchor
• Major & Minor Protection
  
  Movement

Under Construction!

Please note that some people are in fact meanies and will attack you. Stand strong knowing that it doesn't mean you're a bad player and will never be good. It's not always your fault. Sometimes people don't understand how a mechanic works (such as the last boss of Ruins of Mazzatun, who attacks a random player despite being taunted) and will blame you. Expectations are high for a tank to not die, be a group leader, and know strategies, but if you fail to meet a group's expectations that's okay-- we've all been there in one way or another. If you don't know how to tank a fight, watch videos, read guides, or run the content as a DPS or healer and watch what the tank does (though not all tanks will have a good strategy).
If you're coming from being a DPS or healer, that's great! The perspectives of how other roles think really helps with understanding how to be a good tank.

How to get gear?

How to pull trash?
I know it's fun to use a gap closer in trash but it's not necessary (also please never use Dragon Leap... it will scatter the enemies all over-- direct opposite of what you're trying to do.
Run ahead of your group towards the trash. Start by taunting the biggest adds or, if there are none, the one in the center. Drop your AoE debuff or slow if you're using one (such as Razor Caltrops) and Area of Effect pull in the trash. Immediately immobilize them. You can now do any additional buffs and debuffs (ex. applying Major Vulnerability) while you chain in any trash that didn't get pulled by your Area of Effect pull.

• Allowing Flanking - Try to turn the boss so that it's facing away from the group whenever possible. This allows players who have the Master Assassin passive to gain bonuses. This is also good because many enemies will have a "cleave" attack (such as a ground-area of effect in the shape of a cone) that you do not want to hit your teammates. If people stand behind you after you make an effort to let them not do that, then it becomes their problem and you could inform them not to or just wait until an enemy (if any have cleave) kills them.
• Line of Sight (LoS) & Outranging - Larger enemies generally cannot be chained but they will try to move so they can see and attack you, so long as you have taunt. You can use this to get them to move by hiding behind objects or running away from them-- useful for when there's multiple large ranged enemies (Black Gem Foundry trash, 2nd to last boss in Banished Cells 2) or a ranged enemy you'd like to move for some other reason (last boss in Fungal Grotto 2).
• Stacking for Cleave - The sooner things die, the easier it is for you. Try to get as many enemies grouped together as possible (chaining in enemies to a boss, moving towards stationary enemies, etc.)
• Staying Still - The less you move, the better. If you spend the fight running around, DPS have to keep placing their ground Damage Over Time abilities and healers have to keep placing their Healing Over Time abilities, heavily reducing their effectiveness. This also means enemies will move out of your ground-based abilities as well, such as Frost Blockade.

Under Construction!

Enchants can only be afflicted to one enemy at a time.

There are a few options for weapons depending on who you are and what you're doing:

• Ice (decisive or charged) / Ice (infused) - Ideal for Minor Brittle uptimes. If you decide to do endgame trials, you should be able to do this. Do not use Trifocus unless you're making a meme magicka tank build.
• Sword and Board / Ice (infused) - Old style tank setup, lets you use void bash or master's sword and board. Trifocus can be used to split up your blocking resource (stamina on shield bar, magicka on ice bar). The use of trifocus is highly controversial but if you'd like to use it, there's nothing stopping you in dungeons.
• Sword and Board or Ice / Daggers - An option for sweatier groups where you're trying to squeeze in damage as a tank, have to have the skill to not block heavy attacks on the incorrect bar.