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.

Health
"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.

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 enemies that can overwhelm a low HP party) or a powerful boss. Attracting monsters to attack a high AC target not only increases survivability but also conserves healing SP considerably in longer quests or raids.

Saves
When tanking bosses which cast spells (Nearly every boss in the game currently) 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. As well as enough fortitude save to not get killed by commonly casted very dangerous spells like Disintegrate. Willsave is important as well, however almost all important willsaves in the game can currently be negated by certain spells or equipment, so there are alternative options to possesing a massive willsave.

There are multiple sources and categories of AC. Typically, having enough AC for engame elite gameplay is very costly both in terms of time investment for certain items as well as platinum cost. Having enough AC to have any real impact tanking named enemies in endgame epic is typically impossible.

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.

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.