Get to the end of it, and kill the boss. Pretty much the jist of it.
Depending on your group, this can be easy/hard.
Warrior = Takes all the damage of the group.
Healer= Heals the Warrior (A good warrior wouldnt get any other players hurt.)
Ranged DPS: Deal's damage to the target, also control's "add's" (Bad guys that walk into the fight)
Melee DPS: Kills shit really fast (My job ^__^)
Some random class.