Druid

The Druid is a shapeshifting hybrid class, and is also one of the most versatile classes in the game. It is the only class that can fulfill all four roles in the game: Tank, Healer, and melee or ranged Damage Dealer. As with every other hybrid, Druids need to specialize to be effective in any role. Due to their need to shapeshift to fulfill roles, they can only perform one role at a time. Druids must switch between the different shapes in order to adapt to situations as they come.
WoWWiki