Tank

A tank is one of the five main types of character builds, alongside melee DPS, caster, healer, and Rogue, with a primary focus in either high HP or high AC in order to maintain aggro for the party. Their role is to draw enemy monsters to attack them during combat so less durable but higher damage party members can attack and win by gaining high Threat or Hate for proper Crowd Control.

Many of these raid bosses almost require a tank in the party to complete the raid.

Health
Non-Warforged "Flesh" tanks will especially push this attribute, with either high Constitution, taking the Toughness feat multiple times, or having HP-boosting items. While being able to absorb more hits is always helpful, it is also highly dependent on being able to heal, whether by yourself or from another party member, or your HP advantage will vanish. (Remember that shrines will only heal as high as your Heal skill will allow without a healer nearby.)

Armor Class
Better than being able to take hits is not being hit at all, especially when dealing with either "trash mobs" (large groups of low CR melee enemies that can overwhelm a low HP party) or a powerful boss who can instantly kill weaker party members. Attracting monsters to attack a high AC target not only increases survivability but also conserves healing SP considerably in longer quests or raids.

There are multiple sources and categories of AC. Having enough AC to tank named enemies in endgame Epic is close to impossible without being True Reincarnated, going on plenty of loot runs, and spending considerable amounts of platinum.

Saves
When tanking bosses which cast spells (nearly all in the game), saving throws is far more important then armor class. Having enough Reflex Save to half or negate (if you have the Evasion feat) damage spells makes a huge difference in your overall survivability. A high Fortitude save is key to avoid being killed by commonly casted dangerous spells like Disintegrate. Will save is important as well. However, most important will saves in the game can be negated by certain spells or equipment such as Death Ward, a worn Deathblock item (against Beholders), Protection from evil against Command and so on.

(The Resilience feat is useful for its +4 stacking saves when dealing with traps or closing in on an enemy caster. Be sure to turn the combat mode off when attacking.)

Damage Reduction
While not as important as health or armor, damage reduction can be a deciding factor if a party's healer is incapacitated or otherwise occupied between victory and a wipe. There are numerous sources of DR although they do not stack and will not be a substitute for either HP or AC in lategame or epic gameplay.

Intimidate
Since it can be used on an entire cluster of monsters, this skill is one of the best ways to properly manage monster aggro especially to offset the lower damage (and Hate generation) of a tank character. This skill is best used with a shield equipped and will be the most-used button on a tank's hotbar.

Balance
Many enemies have considerable DCs to their Knockdown or Trip attacks and having a considerable rank in this skill makes it easier to get back up from being prone if your Strength Save roll fails, which may be often if you chose not to max strength in favor of other attributes. Typically, a tank should have at least 15 Balance by Level 20 or more.

Enhancements and Clickys
Where your build is deficient, using active enhancements (to boost attack, damage, or saves) and "clicky" items can be the difference between completion and an unrecoverable party loss. Most tanks will have lower attack and damage than melee DPS builds, so having action boosts are needed to do damage for capturing monster aggro.

Items or potions that can cast Divine Favor, Heroism, Lesser Restoration, and Barkskin (if you're not a Ranger) allow your tank to be more durable without draining party member SP. If several of your party members begin dying (especially your healer), use Solid Fog and/or a speed clicky (be it an action boost or Haste) to quickly break contact with monsters, grab members' Soul Stones, and retreat to a Resurrection Shrine.

Preferred Races
Warforged are the preferred race for tanks due to their many immunities and ability to be healed by both healers and casters with Repair spells. They receive enhancements to boost Constitution, DR, and bonus resistance to Will-based enhancements. Bonuses to healed HP, such as Healer's Friend, are absolutely required however. However, they require a VIP/Premium account to unlock and use armor separate from the other races, in the form of Docents.

A Dwarf, while not possesing as many immunties, is very resistant to spells and poisons as well as having an innate bonus to Balance. They are equally tough as Warforged since they possess the same bonus to constitution and are more easily healed. They can have significantly higher blocking DR with a shield as well as access to the Dragonmark of Warding.

Other races are also viable including Half-Elf (Two separate Intimidate timers and Dilettante feats), Elf (Increased Dexterity), Drow (Increased SR and Dexterity), and of course Human.

Preferred Classes
The most popular tank class paths are Frenzed Berserker Barbarian, Paladin Defender of Siberys and Fighter Stalwart Defender. However, really any class can tank given they have enough of the above defenses.