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.

There are multiple sources and categories of AC. Typically, having enough AC for lategame or epic gameplay is very costly both in terms of time investment for certain items as well as platinum cost.

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 Knockdown or Trip DCs and having a considerable rank in this skill makes it easier to get back up from being prone if your Reflex Save fails, which may be often if you are wearing heavy armor as a tank. 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 as hardy, is very resistant to spells and poisons as well as having an innate bonus to Balance. They are more easily healed and will have significant 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 Paladin Defender of Siberys and Fighter Stalwart Defender. However, there are alternatives including Favored Soul, Cleric, or even Bard.