The Druids of Sanctuary fought bravely in the war against Diablo and his minions, but are not going to be presented as playable class in Diablo III.
In the ancient tome of the Druids, the Scéal Fada ("Long Story"), it is written that Bul-Kathos, the great and ancient king of the Barbarian tribes, had a mysterious but trusted confidant, who was only referred to as Fiacla-Géar ("Sharp-Tooth"). This man is sometimes described as Bul-Kathos's close friend, and at other times, he is referred to as his brother. Whatever the source of their relationship, their bond was strong, and together they shared the secrets of the ancients: of the mysteries below the peaks of Mount Arreat, of the venerated task put to their people to protect those mysteries, and of the prophecies regarding the dark times ahead. They both agreed that, in order to fulfil their sacred trust, their people must devote themselves to nothing save that charge. However, they disagreed on exactly how the people could best do this. Bul-Kathos believed that only by bringing the tribes together and training them in strict martial discipline could the tribes faithfully concentrate on their goal for generations to come, known as the Children of Bul-Kathos - the Barbarians.
Fiacla-Géar, on the other hand, believed that only through obtaining a spiritual oneness with the land they had sworn to protect could the people truly appreciate the importance of their role. Both agreed the other's philosophy had merit, and so around the time Bul-Kathos united the tribes, Fiacla-Géar gathered together a small group consisting of the tribes' greatest warrior-poets and shamans and mysteriously retreated into the forests surrounding the area known as Scosglen.
Some sources mention that this separation happened around the time of the Mage Clan Wars, and that the Druids were banished by their Barbarian brothers. Both stories could well be true, and the separation of the two people being under an agreement with the two brothers, but also happening at the same time as fear of the corruption of magic had spread among the noble warriors.
Scosglen and the Druids
They ended up in the harsh wilds of Scosglen where Fiacla-Géar and his people created the first of the Druid Colleges, huge mortar-less stone towers, covered in vines and safely hidden beneath the leafy canopies of the dense forests. They have dwelt there ever since, forging a new way of life for themselves. They created a new culture and language, cutting themselves off from their Barbarian cousins and their ways, vowing not to return to the Steppes of Mt. Arreat until the time of the Uileloscadh Mór, ("Great Holocaust") between the men of the world and the demons of the Burning Hells.
Here, in preparation for the impending conflict, he taught his people the Caoi Dúlra ("Way of Nature"), a harmony with the natural elements of the world, its plants and animals, as the heart of its most intrinsic beliefs, for they are the personification of the very world that the Druids have sworn to protect. Not only was Caoi Dúlra the basis for their system of values, but through its study and practice, the Druids learned to bond with the natural entities of Sanctuary. This bonding was so absolute that eventually they discovered how to talk to the plants and animals, and these beings taught them all the secrets of the natural world. They taught them methods to call animals from far away, how to summon sentient plants from the earth, ways to change their own forms to share the strengths of their animal cousins, even, to a limited extent, techniques to control the weather.
This form of magic also seemed to be lacking the corrupting aspects that Arcane and Demonic magic holds, one of the greatest fears of the Barbarians.
At the Túr Dúlra ("Tower of Nature"), the greatest of the Druid Colleges, stands a great tree Glór-an-Fháidha ("Glory of the Oak"). This tree is the most revered source of the Druids guidance and teachings. Under its branches, for centuries, the Druids of Scosglen have been honing not only their powerful arsenal of natural magic, but also the martial skills they have retained from their Barbarian forefathers. They have done this because they believe themselves to be the world's last line of defence when the time of the great conflict comes, a time they believe is at hand. Lashing out in fury at the recent insurgence of denizens of the Burning Hells, and at the Leathdhiabhala ("Half-Demons"), demonic corruptions of the very creatures they have vowed to defend, the Druids have, at last, emerged from their forests, marching toward their final stand against the minions of Chaos.
Perhaps the most notable power of these Druids is their ability to change their form into that of an animal. Because of this ability, the tribes under their rule knew that they could be watched at any time. The sight of what was thought to be a common beast transforming into a vengeful Druid kept many a potential uprising in check.
The Druid controls both Elemental and Prime magic. His Elemental powers give him control over fire, earth and the winds. His Primal bond with nature allows him to command many beasts and plants, summoning hawks, wolves, vines, and even natural spirits to do his bidding. He can also change his own shape into creatures such as a huge cave bear, or a dire wolf, and then use these forms to call upon further powers.
The Druids are a race of nomadic warrior-poet-kings. Driven from their homelands long ago by their Barbarian brothers, the Druid tribes live primarily in the northern forests. Using mystic secrets passed down through the generations, they summon the elements of fire and wind to do their bidding, and command the creatures of the forest to aid them in battle. Shifting from their human forms into that of wild beasts, gives them abilities far beyond those of other mortals.
Diablo II Info
This is just part of the material available on this topic. The Diablo2Wiki Druid article is available in the Diablo 2 wiki, which has a specific Diablo I and Diablo II focus. A lot of information unique to the first two Diablo games is available there. You can find it here:
Druids use names and phrases from our own ancient Irish language. Whether or not we'll see more of that in Diablo III remains to be seen. Here is a snip from Dragonhelmuk's Druidic Lexicon: 
Túr Dúlra - Here our "natural" Dúlra again is this time describing the "greatest of the Druidic colleges" and in Irish Túr is a tower, so we have "Natural Tower". (Personally I would have gone with the elegance of a genitive construction rather than a simple adjectival description, but then I guess that just shows my lack of appreciation for the simple Druidic names... or something. ;>)
Also, one that isn't mentioned on this lexicon: Dubhdroiacht - demonic magic, literally Black Magic (Dubh Draiocht).
A map of Sanctuary was given as part of the pre-release information about Diablo III, and at the time it came out I didn't look at it much. A few months later I came back to it however, and although Túr Dúlra is mentioned on the map the spelling has been changed. For a moment I wondered if they'd decided to make the spelling look older (as would befit druids) or changed the name for a particular reason... The new spelling was "Tur Dura" and other than trying to use an adjective in a noun genitive that would translate most easily as either "Tower of Durability" or "Tower of Dourness"... Perhaps they were trying to give a hint about the fate of the druids!
Unfortunately when I went to check the map online I saw that the spelling had been changed back to it's original form from Diablo 2 since my personal copy of the map was made. (Actually the new version said Tūr Dūlra, but I think that's just the Diablo font.) - So the Blizzard team were probably just lax... Or were they?! Either way it's nice to see that Túr Dúlra is still a landmark in the Sanctuary of Diablo 3. Who knows it might even be a visitable location like the glowy places on that map! Perhaps all this Druidic will come in handy yet afterall.