Linguists disagree about the exact source and meaning of the word druid, but the original meaning seems to be "immersed in knowledge". When the Celts began their expansion around 1000BCE the learned among them were called "oak knowledgeable". All the tribes had a sacred tree at the center of their territory. Oak veneration continued among the Celts long after it had been abandoned by co-existing cultures.
Julius Caesar stated that Druidism originated in Britain but modern historists believe that Druids were found thru-out Celtic society unrecognized as an organizing body until the 2nd century BCE. Classical writers recorded what little we know of historical Druids. These writings were from peoples in direct competition with the Celtic tribes and most are written from a prejudicial viewpoint. Still, it is from these writings that we have learned the Druids were judges, overseers of religious practice, advisors to kings, diviners, poets, teachers, astrologers and philosophers.
The Druidic/Celtic religion was eventually overcome by the Christian religion. The Picts in Scotland were one of the last Celtic tribes to convert, around 600CE. Many of the customs survived as folklore, and there is speculation that "bardic schools" continued teaching many of the old ways.
A Druid romantic revival began in the 1700's and continues today thru many New-Age philosophies. While many of these are valid spiritual paths, the practitioners of such should remember that the modern descendents of the Celts and Druids are struggling for survival as a civilization. When the Celtic languages disappear so too will the culture that has existed for 3000 years. Many Druid trads and Celtic reconstructionists recommend learning a Celtic language. It is thru language that a culture lives. We should not focus on ancient traditions and stand-by while the remaining culture dies.