A tank is one of the five main types of character builds, alongside melee DPS, caster, healer, and trapper (Rogue or Artificer), 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.
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 a higher number of HP is always helpful, it is also highly dependent on being able to heal, whether by yourself or from another party member. Without a high degree of healing amplification, your HP advantage will vanish. (Remember that shrines will only heal as high as your Heal skill will allow without a healer nearby.)
Damage absorption allows a player to completely ignore a fixed percentage of all incoming physical damage. Damage absorption is calculated from Physical Resistance Rating - the higher the rating, the higher the percentage of damage absorbed. Damage absorption is applied after damage reduction.
Damage absorption works regardless of your enemy attack rolls or other factors, making it extremely powerful of any strong tank.
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.
Unlike in the PnP version of Dungeons&Dragons, every point of armor class decreases the chance of being hit by an enemy, no matter how high your armor class already is. The higher the armor class, the smaller the chance, although the exact chance depends also on the enemy attack bonus. In general, the expected number of attacks a player can survive without healing increases linearly with the armor class.
When tanking bosses which cast spells (nearly all in the game), saving throws are far more important than armor class. Having enough Reflex Save to halve 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 cast dangerous spells like Disintegrate. Will save is important as well to prevent being held or feared. 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 casting.)
While sometimes 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 two types of DR to keep in mind. The first is the passive type displayed on your character sheet can come from being Barbarian or Fighter DR, a Warforged's body, or items. Passive DR is usually at most 6 (higher with Warforged Favored Soul capstone).
The second type is activated only when actively blocking with your shield (holding the SHIFT key) and is much more considerable, surpassing 30 DR with the proper gear. Note that blocking DR works during shield bashing but may not work if flanked or attacked from behind. "Turtling" with a blocked shield in tough fights with proper aggro can reduce a mob with lethal physical damage to an easily manageable fight that will not tax healing.
There are numerous sources of DR although they usually do not stack and will not be a substitute for the other defenses in endgame/epic gameplay.
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 a frequently used button on a tank's hotbar allowing you to rise to the top of a monster's Threat list.
Bear in mind that your character must be able to do a proper amount of damage (with hate generation bonuses) to hold aggro after Intimidate or Ki Shout is used or it may attack other party members.
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 to get back up within one roll.
The most common and effective tank setup is a one-handed weapon (typically slashing) with a shield. Characters, especially fighters with the proper feats, can equip a Khopesh with Tower Shield for maximum damage and protection, but most weapons or shield types will do. A shield also does Bludgeoning damage when bashing, which makes it useful for skeletons, some constructs, oozes, and other monsters. (Adamantine or Everbright shields can be helpful in these cases.)
Keep in mind that the larger the shield, the lower your Dexterity bonus to armor and the higher your armor check penalty to skills such as Balance, Jump or Swim. Mithral shields are prized for this reason as they lower both penalties while retaining the bonuses to AC and DR.
Monk-type tanks will typically be using handwraps although Kamas or other Ki weapons can work. Handwraps are preferable because they allow your character to use Stunning Fist which holds an enemy in place.
Two-handed weapons or even dual wielded weapons can work, especially for grabbing attention from multiple monsters at once with glancing blows or Cleave, where Intimidate is insufficient. Wands or ranged weapons are not recommended unless as a last resort or to attack monsters inaccessible from melee range.
Enhancements and Clickies
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.
Warforged and Half-orcs possess the Brute Fighting enhancement which enhances their Hate by a stacking 25% when activated.
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.
Arcane hybrid tanks can find a great amount of use from Dimension Door especially for dealing with wipes or using a previously saved shrine in the dungeon.
Warforged are a great choice 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, fortification, DR, and bonus resistance to Will-based enchantments. Bonuses to healed HP, such as Healer's Friend, are often required however as most groups rely on divine healing rather than arcane. Warforged will require a VIP/Premium account to unlock and use armor separate from the other races, in the form of Docents.
The Dwarf is very resistant to spells (up to a +5 bonus) 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 much more easily healed. They can have significantly higher blocking DR with a shield as well as access to the Dragonmark of Warding.
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.
See also: Aurrorian's Guide To Being A Tank (2011)