Our historical knowledge of the druids is very limited: much oral material is claimed to have its origins in antiquity, but there are also (problematic) classical sources in Latin and Greek. According to many of these, the druid teaching was split into three "grades":
There are a great number of Druid groups in Britain, Europe and America, with varying claims to (and interest in) the historical traditions.
See also: Stonehenge, Order of Bards, Ovates and Druids (OBOD).
In role-playing games, a druid is a character that represents a magic-user who is most comfortable in natural environments (especially forests). They work with nature to achieve their goals. Druids are related to rangers, except a ranger is more oriented towards traditional combat whereas a druid is more magically inclined. Some RPGs have a shaman class that ends up with similar abilities but is designed around the idea of a medicine man in tribal societies.