The hardware consultants of the stone-circle building industry, as found in The Light Fantastic. The premise is that the stone circles and the like were early computers, used to tell what was happening in the universe. It was "always cheaper to get a new 33-Megalith Circle than upgrade your old one" (very clever - a Megalith is a large stone which has been used to construct a structure or monument, but the reference is clearly to megahertz/megabytes, computing terms for CPU power). TP very neatly explicates the heretofore unexplained mystery of the movement of enormous stones from far away in order to create the stone circles/calendars/computers (and, incidentally, uses the same argument during the construction of the pyramid in Pyramids, confounding L. Ron Hubbard fans everywhere): the druids don't have to convince tens of thousands of people to march eighty miles, carve out a large number of monoliths that weigh upwards of fifty tonnes and drag them on rollers eighty miles back. No, no - they simply cast some form of spell over them and then fly them through the skies to land gently in the right place. Genius.
Druids are also something like the Tezuman priests in that a focus of belief is the shedding of human blood. Virgins such as Bethan are raised and kept pure until the correct time (as decided by the Druids themselves, obviously) and then the locals get to witness the cut and thrust of religion at first hand.
Druidism is also the state religion of Llamedos, a country where faith is taken very seriously. And strictly. While stone circles and human sacrifice constitute a large part of Druidic faith, Llamedos adds its own refinements, such as the dread reinforced battle-harps, whose sonic resonances can do terrible things to an enemy army, or indeed anyone standing within three hundred yards of the vibrating strings. The male voice war choir is also something to behold: the old barbarian standard "We're Going To Cut Your Tonkers Off" becomes truly terrible when done in fully conducted harmony with a descant and everything. (In fact, it could be sung, with descant, to the chorus of "Cwm Rhondda / Bread of Heaven", which is an interesting thought). The Compleat Discworld Atlas implies that Druids can raise real power at need, and are capable of relocating mountains and the very necessary consequent business of pacifying, or even defeating, extremely annoyed trolls who either wonder where their home has gone to, it was here a moment ago, where my bloody mountain gone, is dat to do with you humans in der nighties? Or, in the case of the older, larger and wilder trolls who can be mistaken for mountains, dealing with them after they get translocated.
Llamedos' druidism also insists the country close down totally on holy days and sabbaths, to the extent that the best place to get a drink in Llamedos on a Sunday is in a neighbouring country.