Druid is a creature type.
They are those who associate themselves with the primal forces of nature.
In the early days of Magic: The Gathering any creature that used nature magic was called a druid. The creature type was not well defined and included more or less every green magic-user. Not too long ago it was decided that druids are any and all humanoid creatures that are associated with making mana. Because of this Llanowar Elves gained the creature type Druid and the new card Exuberant Firestoker is also a Druid despite being a red card.