Druids worship nature and occasionally practice some form of animism. Their unique connection allows them to cast holy magic as normal, without causing them to go insane (trees rarely cause anybody to go anything, let alone mad). However, becoming a Druid is not as easy as saying “I love nature! It’s neat!” The Druids form a closed society in the wilderness and are largely regarded as powerful savages who attack any forms of civilization which attempt to trespass on their territory. As a rule, they are unwilling to aid non-Druids. Rangers serve as the middlemen between Druids and civilization when necessary; they are treated with suspicious tolerance by the Druids. Different pockets of Druids exist in different areas of the wilderness, but they all consider themselves part of the same group, under the same centralized leadership.
Most folks in civilization think of Druids in the same way the average Roman citizen thought of the Germanic peoples: wild savages who are best left to children’s tales.