Every character plays a crucial role when doing a dungeon, raid or even questing as a group.
Here is a list of classes and thier roles...
WARRIOR-TANK (Tanks are always first to attack target). When a warrior charges a group of creatures he should always use sunder as his first move to keep aggro on him...
DRUID-TANK, OFF TANK, HEALER (Off Tanks catch aggro that tank does'nt). Druids have high armor in bear form.
PALADIN-TANK, OFF TANK, HEALER (Same as warriors and druids).
HUNTER-DPS (Hunters are damage per second). They should use traps to slow enemies and pets to pull aggro from healer.
ROGUE-DPS (Dps and sapping targets). Rogues are good for sapping/stunning creatures before a battle to immobilze them.
WARLOCK-DPS (Dps and off tank pets). Good for Dps, banish demons, enslave demons and pulling aggro from healer.
PRIEST-HEALER (Heal party members). Priest's are also good for shackling undead, mind control and psychic scream.
MAGE-DPS (Dps and slow attackers). Mages are also good for sheeping a creature, and frost effects.
SHAMAN-DPS, HEAL (Dps and heal party members). They should use totems and slowing the enemy.