Druids were considered the priests of the Celtic peoples.
Managing a precarious hold between the lessons of wisdom and philosophy of the matriarchal cultures and the practical knowledge and historical information of the Celtic tribes and myths, the druids were some of the most respected scholars and holy men of the ancient world.
Druids acted as priests during ceremonies and were considered to have supernatural knowledge not unlike that of a shaman. They were exempt from all military and political responsibilities.
The oak was considered to represent maleness because it grew straight and was immensely useful wood. The mistletoe's sap made it a symbol of virility, and a barren woman who drank of it would become fertile.
Very few aspects of the druids are consistent and have been agreed upon by modern scholars. Among them, that they were religious leaders, that they held the oak and mistletoe to be sacred, they believed in mythology, and that they studied astronomy and natural philosophy. The druids were persecuted by the Romans in the first century and do not appear again in the historical record until the Christian missionaries of the fifth century. There they are categorized merely as sorcerers who unsuccessfully opposed Christianity.