Holy Trinity

From WoWWiki

The most tried-and-true way to build an ideal group consists of the following Holy Trinity:

• Tank(s) to take the damage (Warrior, Druid, and Paladin.)
• Healer(s) to keep party members alive (Priest, Druid, Shaman, and Paladin.)
• Damage dealers to deal most of the damage (Mage, Warlock, Hunter, and Rogue are pure damage dealers but the other five classes can be damage dealers as well with the right spec and gear.)

Having each of them covered is not the only way to survive, but many people will initially try to build such a group when setting out for instance runs at high levels.