In the
1e PH it says "Druids can be visualized as medieval cousins of what the ancient Celtic sect of Druids would have become had it survived the Roman conquest. They hold trees (particularly oak and ash), the sun, and the moon as deities. Mistletoe is the holy symbol of druids, and it gives power to their spells. They have an obligation to protect trees and wild plants, crops, and to a lesser extent, their human followers and animals."
In
Greyhawk they were associated with "The Old Faith."
In
Forgotten Realms they have to have a patron nature god.
In the
2e PH it says "Historically, druids lived among the Germanic tribes of Western Europe and Britain during the days of the Roman Empire. They acted as advisors to chieftains and held great influence over the tribesmen. Central to their thinking was the belief that the earth was the mother and source of all life. They revered many natural things—the sun, moon, and certain trees—as deities. Druids in the AD&D game, however, are only loosely patterned after these historical figures. They are not required to behave like or follow the beliefs of historical druids.
The druid is an example of a priest designed for a specific mythos. His powers and beliefs are different from those of the cleric. The druid is a priest of nature and guardian of the wilderness, be it forest, plains, or jungle."
2e also had the
Complete Druids Handbook which provided a bunch of druid kit specializations for different terrains.
There was also
HR3 The Celts Campaign Sourcebook, which altered druids to be more historical-mythic Celtic appropriate.
Dark Sun had druids connected more to natural areas and terrains as there were no gods and clerics were connected to elementals.
In
3.5 the PH said "A druid reveres nature above all. She gains her magical powers either from the force of nature itself or from a nature deity. The typical druid pursues a mystic spirituality of transcendent union with nature rather than devoting herself to a divine entity. Still, some druids revere or at least respect either Obad-Hai (god of nature) or Ehlonna (goddess of the woodlands)."
3.5 also introduced
Eberron with different traditions of druids starting with a black dragon founder and orc gatekeepers who fought an ancient aberration incursion and follow an animistic religion.
Faiths of Eberron discussed multiple druidic sects including the fanatical anti-arcane magic and civilization Ashbound, the apocalyptic Children of Winter, the aberration hunting Gatekeepers, and the fey relationship Greensingers.
4e had druids from
Player's Handbook 2 as connected to the Primal Spirits, a later addition to the World Axis Cosmology. "Religion: Most druids do not worship the gods of the Astral Sea, instead orienting their lives and beliefs around the primal spirits of nature. It might be misleading to call their relationship with these spirits worship, but druids invoke the spirits in times of passage, implore them for aid, and appease them with offerings of burned food or spilled blood."
The 5e PH says: "Whether calling on the elemental forces of nature or emulating the creatures of the animal world, druids are an embodiment of nature's resilience, cunning, and fury. They claim no mastery over nature. Instead, they see themselves as extensions of nature's indomitable will.
POWER OF NATURE
Druids revere nature above all, gaining their spells and other magical powers either from the force of nature itself or from a nature deity. Many druids pursue a mystic spirituality of transcendent union with nature rather than devotion to a divine entity, while others serve gods of wild nature, animals, or elemental forces. The ancient druidic traditions are sometimes called the Old Faith, in contrast to the worship of gods in temples and shrines."
