Druids, what were they? Some call them priests, others philosophers. Possibly they were healers, metal workers, linguists or even shaman. Well in truth no one can say. You see the Druids were oral traditionalists, so nothing they did or taught was ever written down. We do have references from the writers of classical times, but they have there own cultural biases. So one thing we do know is that we don't know much.
We can however use the written history we do have to make some assumptions, as long as we acknowledge this and don't pass this of as fact as many who write about Druids do. We can also use myth and still practicing pagans as a possible reference. When I say still practicing pagans, I mean people who still practice in places the Celts came in contact with on their migrations. This would include India. It is thought by many neo-pagans that study Celts and their religion that the closest thing to an original druid ritual would be a Vedic fire ceremony.
Folk dances and traditions could also provide us with left over rituals, like the hobby horse dance in Cornwall, and Morris dancing.
Our other avenue for information is archaeology. Not to say that this hasn't given us misinformation in the past. In Britain and France, when an old temple was uncovered it was usually because someone was looking for a Roman temple, which was often built on an old Celtic temple. This would have received hardly any attention from those digging and been considered as somewhat of an inconvenience for those trying to find out some history of a "civilized" people.
Things are now changing and the Celtic people are enjoying their true place in history. The fact that anything Celtic is "hip" now is to the Celtic scholars' benefit, as people seem to have an insatiable appetite for all things Celtic. That means new digs in Europe are focusing on the Celtic remains instead of the Roman. This will mean a whole bunch of new archaeological information coming out in the next few years.
So that still leaves the question of who were the Druids unanswered. The fact is you must draw your own conclusions to who they where and what they did. Be adaptable, that was the Celtic way. Read books by anthropologists, self professed Druids and other pagan publications on the subject. But remember the Celts were not a group of nice pagans doing a ritual in the woods. They were a primal primitive people who did things we consider improper these days. So don't be a new age wimp if you want to be a Druid.