In simple terms, the Druids were the priests of the Celtic tribes in Britain. But to state that fact does not convey the breadth of their influence in Celtic society. The Druids were a sort of super-class of priests, political advisors, teachers, healers, and arbitrators among the Celtic tribes.
They had their own universities, where traditional knowledge was passed on by rote (i.e. memorized). Druids had the right to speak ahead of the king in council, and may in some situations have held more authority than the king. They acted as ambassadors in time of war, they composed verse and upheld the law. They were a sort of glue holding together Celtic culture.
We know that the Druids used both animal and human sacrifice, and that many of their observances centred on oak groves and water. The Isle of Anglesey, in present-day Wales, was a centre of Druidic practice.
The Druids as we know them today exist largely in the words of the Romans. The trouble with the reports of the Romans is that they were a mix of reportage and political propaganda. It was politically expedient for the Celtic peoples to be coloured as barbarians and the Romans as a great civilizing force.
The Romans seem to been genuinely horrified by the instances of human sacrifice among the Druids. In 61 AD the Romans exterminated the Druids of Anglesey, effectively destroying druidism as a religious force until a form of druidism was revived in the 19th century.
Related:
Celtic Britain
Roman invasion
