What did they do? Who did they worship? We don't really know, because their
enemies, the Romans, who were the only ones to write down any history about the
Druids, gave them bad press. Some of the deities were taken over by the Romans,
who had a knack for that (which is why the Roman Catholic Church has so many saints who were really pagan gods such as St Brigid and St George). There was Curnones, the stag-headed god, and
various mother-goddesses, such as the one who was preempted in Bath by Minerva
(her name was Sulis*). Druids also sacrificed people, probably captured enemy soldiers, by burning them in wicker cages, on the solstices. They were also the doctors and lawyers and historians (bards) of Celtic society, but never wrote anything down, because part of their training was building up prodigious memorization of ancient facts. Reincarnation -- hardly in the Buddhist sense -- was part of their religion.
Modern Druids really haven't got a clue...
Druids, schmuids.
* This is interesting, the fact that spas -- hot mineral waters -- have been associated with goddesses of healing for thousands of years and have never fallen out of fashion, even though the fashion of them has changed with the times. Could it be that the spas really work?