Druids are a shapeshifting Hybrid class. Unlike other hybrid classes, Druids do not fulfill several roles at once (e.g., both healing and melee at the same time), but can choose which role to take by shifting into one of their forms.
In their normal form, they are healers and casters, with a wide array of heal over time spells and some offensive spells. In [Bear Form], they gain considerable toughness and a Rage bar, allowing them to act as a tank. The [Cat Form] gives them an Energy bar and Rogue-like abilities for a high damage output, while their [Moonkin Form] (balance talent) allows them to gain extra armor equal to plate, give their group a spell crit aura, and cast potent damage spells on par with a Mage. They also have 3 travel forms: [Travel Form] for land, Aquatic Form for Water, and Flight Form for air, which dramatically increases their speed, and therefore often survival.
The Druid gains more from stats than any other class (e.g. 20 Agility = %1 Critical hit & 1% Dodge, etc.). This allows them to be moderately sufficient in all roles. If a Druid concentrates all of their equipment to suit one of the Druid branches and uses the correct talents, they can be as efficient as any class for a given role. While Druids can only wear Leather armor and Cloth armor, this is offset by their Bear, Moonkin, and tree forms, which increase their armor by a large percentage (180% (Bear), 200% (improved tree), or 370% (Moonkin and Dire Bear) increase on top of base armor); this makes high-level Druids with the right gear the class with the highest armor in the game. Druids are also the only class to have a resurrection spell which may be used in combat, [Rebirth]. However, unlike the other resurrection spells, it can only be used fairly infrequently (5 minute cooldown) as a trade-off to its combat-usability. This, in addition to their heals, tanking and [Innervate], makes a well-played Druid able to save a group from what would otherwise be a guaranteed wipe.
In end-game raiding, Druids historically were expected to heal. This has been changed with the talent change in 2.0 and increased gear supporting the Feral talent tree. Nowadays feral Druids can spend their talent points in such a way that they can be both a melee-damage-dealer (Rogue-type) in cat form and a tank in bear form. No other class offers that much versatility (DPS and tanking in one single talent build), though the two roles require two different sets of gear. Beyond that, Druids can still specialize to become a healer or a DPS caster.
If you enjoy fulfilling multiple roles, the Druid class might be for you.
Thanks to: Gamepedia