I didn't know very much about Druids... except that they were usually allowed into Stonehenge for the Summer Solstice, and they were supposed to worship in groves of oak trees... until I checked out the official website, details of which appear below. Oh yes, I also had it somewhere in the back of my mind that their name was derived from the Welsh (or Celtic, if you prefer...) word "Derw", meaning oak tree....
The Order of Bards Ovates and Druids... The Druid Grove