The word druid names two distinct things, and an honest account keeps them apart: the learned class of the ancient Celts, and the members of the modern movement of Druidry.
The ancient learned class
Among the ancient Celtic peoples the druids were a learned class serving as priests, judges, teachers and keepers of tradition, holding religious and legal authority and undergoing a long oral training, and much of what they knew was deliberately never written, so that they left no scriptures of their own. What is known of them comes largely from outside observers, above all the Romans, whose accounts are partial and hostile, so the reliable knowledge of the ancient druids is limited and much that is popularly believed of them is later invention.
The modern revival
Modern Druidry is a separate phenomenon, a set of movements that arose from the eighteenth century onward, romantic, fraternal and later religious, drawing on the fragmentary and often unreliable picture of the ancient druids to build new traditions of nature reverence and ceremony, some of which are now recognised as forms of contemporary Paganism. The monuments popularly associated with the druids, such as Stonehenge, in fact predate the Celts by many centuries, a connection made by early modern antiquarians rather than by history. To speak clearly of the druid, then, is to distinguish the imperfectly known ancient learned class from the modern revival that took its name and reimagined it, rather than to run the two together as popular usage does.