The Druids were an Iron age pagan priestly class in the Celtic sphere of influence. Like many things ancient, little is known about the Druids, however, they have been the subject of art, books, revivals and speculation as much as any ancient group in ancient non-Mediterranean Europe. They were a class exempt from taxation and military service and had the ability to stop battles from taking place.
Did the Druids participate in human sacrifice? It is likely, however, there is not evidence to prove it. Although the Druids thrived in Ireland, England, Gaul and other parts of Western Europe, by the 2nd century AD, like so many things Celtic, only the British Isles remained as a haven for the culture. Many people say the Druids built Stonehenge, though this is not a fact. Druids possibly acted as soothsayers or as doctors. Healing rituals were likely used in conjunction with medical procedures.
It is also possible that the 'snakes' St. Patrick was alleged to have driven from Ireland had more to do with the symbolic connection of the serpent to the Druids. This would have been around 400 AD when Druidism would have been on the decline, maybe even in Ireland (usually the last region to lose various Celtic elements.)
Today, there are many people in the western world who consider themselves Druids or at least incorporate Druid rituals (though these practices are mostly guesses to what ancient Druids actually did) into a greater belief system or lifestyle.