The elves were the first people to harness the magics of Orea. They enjoyed a mystical capacity inherent to their nature. Some secrets gleaned from their interactions with the gods. Much was learned from the dragons and titans, who worked the powers of the cosmos before even gods walked the realms. The most easily worked by elfish affinities was the magic of the physical world, itself. The green growing things of Llyndra's care, Pehn's mastery over animals, the secrets of the elemental forces that formed creation.
When the Five Tribes of Men were brought to the world, it was the Tribe of Green that were first to establish peace and trelations with the elvish lords and lands and it was to them the first secrets of magic were shared with humanity. To these were shared the songs of the winds and rain, the cycles of the moons, the tongues of beast and bird.
These Men of the Green Tribe, revelled in, to, and with the natural world. Seeking not power over it, but the respect, vitality, and knowledge freely available and inherent to those who knew the ancient ways of connecting with it. They became as the Myrielldiin, those of the Holy [now also called, "Ancient"] Order of Mistwood - the original, most sacred, and largest wooded stronghold of the order. Those who men have come to call Druids.
By the end of the first Age Men, the most senior masters and elders among the Myrielldiin transcended this world, leaving those to whom they shared their secrets to see to the continued protection and healthy administration of the world of Orea. And this they have done, to the best of their ability, through the ages.
The Initiates of Ancient Order revere the natural world, from the plants and animals (including peoples), to the cycles of the seasons, Orea's two moons, and life/death/rebirth. They revere the sun, moons, and the turnings of the stars, the rivers and hills, the mountains, deserts, and all the lands, seas, and skies. However, the Myrielldiin hold no reverence for those personifications, those "Gods of Men," which represent those same facets of the Creation.
They can get along with the adherents and faithful of those deities, who do not hold objectionable views to the natural world. But they do not, themselves, offer worship or require the grace of those entities to work their magic. They know the hiddens ways to interact with fae, shadow, and elemental entities, but do not draw power from them, per se.
"Druidism" works spells of power -"Nature Magic"- by their secret sacred tongue, unknown to all but those intiated in the order. The forbidden language, predating humanity, so powerful and primeival as to cause madness in any uninitiated who overhear it. They channel remarkable powers from the direct forces of the world, those elemental and energetic powers ebbing and flowing around and through the material (and adjacent) planes.
Largely, druidism has been relegated to the lands between cities and large settlements, and those frontier realms more removed from the great citadels and stone cities of nations and kingdoms of Men. In the south lands abutting Mistwood, the great northern wood of the O'Douhn Duchy in the kingdom of Grinlia, and the [formerly elfin] "Green Kingdom" of Mostrial, those of the Ancient Order still command great respect and broad appreciation from the common folk and a good degree of respected (or feared) authority and counsel among the nobility.
The Holy Order of Mistwood works across the continent, commanded by their enigmatic ruling body, known only as the Grove. They hold sites in all manner of terrain as sacred, from the earliest ages of Men (and some earlier than that). These are mostly tended by the Keepers (druids of 7th level or greater). Those Initiates beneath the station of "Keeper" are subject to the directives, instruction, and machinations of those above them in a simultaneously loose but unquestioned hierarchy. Within a Keeper's site/jurisdiction, their word is sacrosanct, regardless of the level of any visiting druid save a Grove member.
All druids take a personal name after initiation. This "Green name" is most commonly a type of animal or sylvan creature, plants, or some other natural phenomenon. Other than Keepers, the Mouth of Mistwood, or the lords and ladies of the Grove, all druids are refer themselves as Brothers and Sisters with their taken Green Name: "Brother Heron," "Sister Sage," "Brother Fen," "Sister Brook," etc... The members of the Grove, the thirteen most powerful druids on Orea, are the "Lord/Lady" of one of the sacred trees, and no other druid can take/hold that name. For example, the Grove's "ruling" member, the Lord of Oaks, is the only druid who has a name referring to oak. Also, (and this is not particularly relevant to "druidism," as such, but a world/setting detail) only the Grove's spokesman in the realms of Men, "the Mouth of Mistwood," is/can be known as "Mistletoe."
The Ancient Order has moved nations, formed peace and beset wars, seen to the protection of the natural world and guardianship of communities -of human and sylvan creatures alike, aided the forces of Law and those of Chaos, promoting the Good at one time, seemingly Evil at another. All of this had been done through the ages, in the name of maintaining the Balance and the Cycle. Combined, the Balance and the Cycle, symbolized in the order's half sun/crescent moon icon, is sometimes referred to as the druidic philosophy/ideal of "the Green." All missions and concerns assigned by the Order to its agents across the realms are in the great service of "the Green," assuring the maintenance of the Balance and the Cycle...for the diminishment or offsetting of the Balance, or stagnation of the Cycle, leads only to the destruction of all Creation...and that would only be the annihilation of what is now called "druidism" and the Myrielldiin.