The Druids' name is etymologically unclear. Some scholars think that it derives from an ancient Celtic word for oak, and this makes sense, as the druids were the priests of oaken groves who harvested the sacred mistletoe, but others derive it from an ancient Indo-European word for wisdom. The issue is unresolved. But the ancient Greeks and Romans knew of them, and observed that the order of Druids originated in Britain and spread to Ireland and Gaul. But we must remember that the when the ancients spoke of Prydain [Britain] they spoke of the whole archipelago of the British Isles, including Albion, Erin, and their associated islands. It was part of druidic lore that to fully study druidry an aspirant must therefore go to Britain, going back to the source, as it were. The Greeks , speculated that the Druids were influenced by the philosophy of Pythagorus, on account of their sharing Pythagorus' belief in reincarnation, but this belief has ancient roots that long antedate Pythagoreanism, so this view is unproven.
Our knowledge of the Druids is limited by the fact that they thought it impious to commit their lore to writing, so all their learning was preserved and transmitted orally, and as druidic studies took twenty years, there was much lore to learn. There were grades of Druidry, and for this knowledge we are indebted to the Irish, who preserved it. The basic grade was bard, and these druids were to commit to heart the vast corpus of songs. We know of the bard Taliesin, who knew the songs of Welsh druidry. Druid robes were colour-coded, with bards wearing blue, and above them the ovates, healers, who wore green. Senior druids wore white. So druids sang, healed and if senior acted as counsellors to monarchs and judged court cases. It is likely that they presided over religious rituals. Roman propaganda put about the legend that the druids used human sacrifice, but although human sacrifice was known in the bronze age Britain Isles it seems to be older than druidry, which probably is a later, Iron Age development. The question of how far druids were involved in human sacrifice is still open.
Julius Caesar met druids in Gaul, but later the druids were to get involved in a fatal clash with Rome. They seem to have objected to the Roman take over of their territories, and they therefore may have had a significant role in the revolt of Queen Boudicca, who objected to Roman behaviour, enslaving British children, stealing her tribe's possessions and raping her daughters. The Druids of the sacred isle of Mona, now also known as Anglesey, took Boudicca's side in the revolt, and the Roman general, the murderous Seutonius Paulinus, sent the ninth legion to overwhelm Anglesey. The "civilised" Romans slaughtered the Druids man, woman and child. At this point druidry falls out of British history. The Roman period simply ignores it, and the incoming Saxons had no druids. But was it really gone? In Scotland and Ireland druids still survived, and it seems possible that druidry was nourished in Wales and Scotland by its presence in ErIn.