Shining Slash: It's dangerous to go alone, take this with you

This column will aim to help new players or people not acquainted with the fine arts of Tanking to gain an understanding of the mechanics, ideas and application of a Tanking class.

Tanks. A character archetype and group role common through MMOs, Tanks hold the front line in combat. Their role is, first and foremost, to soak damage. The one trait of a class capable of Tanking, across the board, is that they have a way to mitigate incoming damage in a serious way. This can be represented by high damage resistance, a high chance to avoid damage altogether (evade, dodge), or a way to stay out of the enemy's reach, and so avoid damage.
This in and of itself does not make a Tank (the last example especially).

The second trait a Tank must have is the ability to keep the enemy focused on them. Aggro, as it is most commonly known, refers to the act of an enemy creature attacking a specific target. When you "have Aggro", the enemy in question is attacking you.

Thus: a Tank is a character who is both capable of a) keeping enemies attacking them, and nobody else, and b) significantly reducing or completely removing all incoming damage to themselves.

Tanks are important to groups for a number of reasons, but most obviously the fact that they don't take as much damage as other players, and so are less likely to die before a heal cast reaches them, and that they will need less healing than any other player. Tanks are meat-shields, at the most base level.
By having a Tank in your party your Healers will take longer to run out of mana, your group will be able to take on more opponents at a time, and you'll be able to challenge and defeat bosses or elite monsters that you wouldn't have been able to otherwise.

At first glance they seem useful, but in many situations they are close to necessary. Consider an elite mob in Aion. It has a particularly large health pool, and will take around a minute to kill for a solo Sorcerer, if they are using their CC (Crowd Control) abilities appropriately. Things get complicated when a CC ability gets resisted, or a second elite is alerted. The presence of lone Healer, or indeed a lone Tank, won't improve the Sorcerer's chances of surviving. A heal or two would be nice, but when the Sorcerer dies in one or two hits from even one elite, no amount of healing is going to save his life.

Add a tank AND a Healer, and suddenly the group can chew through elites in half the time it would've taken for the lone Sorcerer, and can take on groups of two or more! With a Tank up front taking the hits, the Healer keeping him from dying, and the Sorcerer dealing enough damage to kill the elites before the healer runs out of mana as well as CCing a third elite as necessary, this group is ready for just about any challenge the game can throw at them. More damage dealers would help, and in rare situations more healers or more tanks, but this three man group is more than capable of, with some smart maneuvering and an eye on surrounding enemies, clearing through elite monsters without too much difficulty.

A Tank can even help speed up grinding or questing in non-elite areas, as they can ensure that even with unluckily pulled additional monsters, there will be no deaths among his or her teammates. This is the role of a Tank, before all others: to protect other players by putting themselves in between the others and those who would do them harm.