Pre-Roman spiritual leaders of Europe, there is much speculation and relatively little factual documentation about them. They seem to have been involved with divination, incantations, healing, magic, advising royalty, carrying history in song. Druidry was an oral tradition, hence the minimal documentation.
Druids were said to meet in groves (especially of oak) and in caves. They were seen as anti-Roman, so Rome (under Augustus) forbade the practice of Druidry and the armies (and later the Christians) would destroy their sacred areas and burn them.
There was a Druid center of learning on the island of Anglesey (Ynys Môn) in Wales. Druids assembled there for training from all over Europe, although the oral secrets revealed there are not known. Rome eventually invaded the island and destroyed the groves.
Today, many of their mysteries are being rediscovered.