When you group in the World of Warcraft there are three sets of characters: Tanks, Healers, and DPS. They are listed in that order, as that is the order of importance in any group. Without a Tank, you can't realistically group. Without a Healer, you may as well not have a Tank. DPS, while important is a very distance third in importance to your main party members.
So when forming a group, you know you need to get a tank, but which one? Which of the four possible tank classes are best? Should you pick the AOE based Paladin or Death Knight, or should you pick the more single target based Warrior or Druid?