Although since Christian times Druids have been identified as wizards and soothsayers, in pre-Christian Celtic society they formed an intellectual class comprising philosophers, judges, educators, historians, doctors, seers, astronomers, and astrologers. The earliest surviving Classical references to Druids date to the 2nd century B.C.
The word "Druidae" is of Celtic origin. The Roman writer Pliny the Elder (Gaius Plinius Secundus, 23/24-79 A.D.) believed it to be a cognate with the Greek work "drus," meaning "an oak." "Dru-wid" combines the word roots "oak" and "knowledge" ("wid" means "to know" or "to see" - as in the Sanskrit "vid"). The oak (together with the rowan and hazel) was an important sacred tree to the Druids. In the Celtic social system, Druid was a title given to learned men and women possessing "oak knowledge" (or "oak wisdom").
The Druids emerged from the ancient Celtic tribes, at a time when the people had to live close to nature to survive. By the light of the storyteller's fire, and with the play of the harp, the Druids dreamed magic for their people. In the deep woods they would gather, bringing together their mysticism and philosophy, their insight and learning. Their spirit emerged from the the tides of the sea, the light of the sun, the wind in the Oak, the cry of the deer. In this way, they created an institution that inspired, frightened, and uplifted their world.
Druids filled the roles of judge, doctor, diviner, mage, mystic, and clerical scholar - they were the religious intelligensia of their culture.
To become a Druid, students assembled in large groups for instruction and training, for a period of up to twenty years.
The mythologies describe Druids who were capable of many magical powers such as divination & prophesy, control of the weather, healing, levitation, and shapechanging themselves into the forms of animals.
Their education was so rigourous that at the end of it they were virtually walking encycopaedias. A good word for them would seem to be "priests", yet I am reluctant to use it for two reasons: The Romans never used it, and because Druids didn't minister to congregations as priests do.
Rather, they had a clientele, like a lawyer, a consultant, a mystic, or a shaman would have.
Caesar and his historians never referred to them as priests, but perhaps they could not recognise them as priests since the Roman priesthood, officiating over an essentially political religion, were primarily teachers and judges, with less emphasis on being seers or diviners, whereas the Druids appeared to have both legal and magical powers and responsabilities.
Some scholars have argued that Druids originally belonged to a pre-Celtic ('non-Aryan') population in Britain and Ireland (from where they spread to Gaul), noting that there is no trace of Druidism among Celts elsewhere - in Cisalpine Italy, Spain, or Galatia (modern Turkey). Others, however, believe that Druids were an indigenous Celtic intelligentsia to be found among all Celtic peoples, but were known by other names.
With the revival of interest in the Druids in later times, the question of what they looked like has been largely a matter of imagination. Early representations tended to show them dressed in vaguely classical garb. Aylett Sammes, in his Britannia Antiqua Illustrata (1676), shows a Druid barefoot dressed in a knee-length tunic and a hooded cloak. He holds a staff in one hand and in the other a book and a sprig of mistletoe. A bag or scrip hangs from his belt.
The main sources we have on what Druids did are the teachings and writings of Roman historians, such data as archeological remains can provide, and mythological literature recorded by monks in the eighth through twelfth century. Also, analogies can be drawn between the Celts and such Indo-European cultures that existed around the same time and had the same level of cultural achievement, such as the Hindu people.
Archaeology is an excellent resource for the study of celtic history. Scientists have uncovered the remains of votive offerings to the Gods in lake bottoms, bogs, and "votive pits" (a narrow hole dug deep in the ground in which votive offerings are buried), which tell us about Celtic religion. There are also the remains of celtic fortresses, habitations, temples, jewelry and tools. These remains speak to us not of events and people in Celtic history, but what life was like, what their technological capeability was, what food they ate, what crafts and trades they practiced, what products they made and traded (which in turn tells us about their economy), and where they travelled and how they got there. These facts about Celtic social life are an important element for understanding Druidism, because it is necessary to understand the whole culture in which Druidism was situated.
Nevertheless they were often impressed by the Druids' grasp of mathematical and astronomical skill. One Roman author, Diogenes, placed the Druids on a list of the ancient world's wisest philosophers; a list which included the Magi of Persia, the Chaldeans (the priesthood of the Babylonians) and the Gymnosophists (an Hindu sect which preceded the Yogis), all of whom were selected for their skill in mathematics, physics, logic, and philosophy.
[Taken from Brittania.com]