This information is not canon, as of 11/14/16.The Druids were formed by Haggar in order to harvest quintessence when Zarkon began his quest to conquer the known universe.
All of the Druids wear long brown robes that conceal much of their bodies, usually exposing only their hands. Aside from Haggar, all known Druids appear to wear white masks on their faces that make it appear as though they have more than two eyes.
All of the Druids possess energy manipulation abilities such as teleportation or producing energy bolts. They are also capable of refining Quintessence into fuel for Galra operations, and Haggar is known for her abilities in casting illusions as well as creating Robeasts. The Druids are also known in the empire for their effectiveness as interrogators and for their experimentation with cybernetics.
The white, bird-like masks worn by Galra druids bear a passing resemblance to those worn by plague doctors in 17th century Europe.
- ...which is very interesting, seeing how Shiro referred to the Galra as "These monsters spread like a plague throughout the galaxy," in the very first episode (The Rise of Voltron).