The tank is a build intended to absorb damage. A primarily defensive build, the role of the tank is to keep more vulnerable party members safe from harm. In your standard fantasy setting, tanks are usually represented by armored melee warriors. Thanks to their strong defenses and endurance, tanks benefit from a generally high survivability. Depending on the game, however, their offenses may be only mediocre, and their range of special abilities limited.
The most effective tanks have significant endurance to damage and a strong capability to defeat enemy attacks. Ideally, the tank also possesses capabilities that can prompt foes to attack it. In MMORPGs, this is often handled by a system of "aggro", where using certain abilities or taking certain actions determines whether the opponents want to attack you. In table top RPGs, though, where the enemies have a thinking person controlling them, the tank needs to provide a good reason for being attacked.
In many cases, party formation is a key point. This is good in a classic dungeon setting, where there is limited room to maneuver. In this case, the tank can stand in front so that powerful melee threats, such as crushers, have to engage the tank first. This is, however, not ideal for playing a tank. Archers, blasters, and other ranged attack builds can often shoot right past a tank at more vulnerable party members, and more mobile attackers, such as skirmishers, can often slip past or sneak by to get at key targets. Likewise, if there is a lot of room to move about, the tank will have trouble defending the rest of the party. In this case, the tank is forced to play the role of a survivable but ineffective crusher.
If you want to play a tank, make sure you are able to get capabilities that make the enemies want to attack you. A good offense is a perfectly viable option; a combination tank/crusher build is a very dangerous melee threat, since it is not only able to deal significant damage, but it takes a lot of punishment to drop. Some games offer "taunt"-like abilities that allow you to coerce foes to attack you in preference to other targets; these are incredibly valuable to a tank. Other options include obtaining abilities that make you a nuisance to the enemy by improving the powers of your party. A tank/healer or tank/buffer is ideal for this (a good tank/healer is especially effective, since its healing capabilities complement its high tolerance for damage).
Blasters are the banes of tanks, since they have the ability to attack multiple opponents at high damage, especially when their foes are grouped together. Although the tank can weather the blaster's attacks, area effects prevent it from doing its job efficiently. Taunt-style abilities can minimize this hindrance by attracting the blaster's attacks and then moving away from the group. The tank is itself an adversary of the crusher, since the tank can hold off the crusher's powerful offenses for long enough for a more offensively-oriented party member to strike it down.
As a defensive build, tanks can sometimes be less enjoyable to play than other character types, since their contribution to the party is relatively passive. However, a dedicated, well-built tank can provide a valuable defense to weaker party members. Tanks are also relatively simple to play, making them a good choice for beginners.
