The Tank's job is to die before the Healer.
The Healer's job is to make the Tank not die
The DPS's job is to kill the mobs and not die.

To be great, communicate, be aware of the surroundings, your abilities, the abilities of the others in your group, and the mechanics. Often as the Tank you will be expected to step into the role of Leader, and the role of Puller, both of which control the speed of the event.