It was John Aubrey, writing in the 17th century, who first thought it a "probability" that stone circles, such as Stonehenge, "were Temples of the Druids" and called his text on stone circles the Templa Druidum. This idea was picked up by William Stukeley, in the early 18th century, who subtitled his first book, Stonehenge, published in 1740, "a Temple Restored to the British Druids, and his second, on Avebury, published in 1743, "a Temple of the British Druids." Although later, in the 19th century, Sir John Lubbock (1834-1913) dated Stonehenge to a period much earlier than the time of the Druids (that is, to about 2000 B.C.E., whereas the Druids don't appear in the historical record until 1800 years later), nonetheless the view was maintained by a minority that Druids were pre-Celtic inhabitants of Britain and that the religious beliefs and practices for which Stonehenge was first built are ancestral to those of the later Celtic Druids. 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.E. The word Druidae is of Celtic origin. The Roman writer Pliny the Elder (Gaius Plinius Secundus, 23/24-79 C.E.) believed it to be a cognate with the Greek word 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"). 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.
Besides observing that the name 'Druid' is derived from "oak", it was Pliny the Elder, in his Naturalis Historia (XVI, 95), who associates the Druids with mistletoe and oak groves: "The Druids...hold nothing more sacred than the mistletoe and the tree on which it grows provided it is an oak. They choose the oak to form groves, and they do not perform any religious rites without its foliage..." Pliny also describes how the Druids used a "gold pruning hook" or "sickle" to gather the mistletoe.
Anything growing on those trees [oaks] they regard as sent from heaven and a sign that this tree has been chosen by the gods themselves. Mistletoe is, however, very rarely found, and when found, it is gathered with great ceremony and especially on the sixth day of the moon... They prepare a ritual sacrifice and feast under the tree, and lead up two white bulls whose horns are bound for the first time on this occasion. A priest attired in a white vestment ascends the tree and with a golden pruning hook cuts the mistletoe which is caught in a white cloth. Then next they sacrifice the victims praying that the gods will make their gifts propitious to those to whom they have given it. They believe that if given in drink the mistletoe will give fecundity to any barren animal, and that it is predominant against all poisons."
This text is taken from the excellent guide book Stonehenge and Avebury published by Pitkin Unichrome, available to purchase