Present day people don't really know the answers you seek. Druids were the spiritually wise people of the Celts. Some would call them priests. They were also advisers to leaders (chieftains or kings). (The information that was written about them, was written by Romans who didn't particularly like the Celtic Druids and considered them to be barbaric <pagan>).
There were three categories of druids:
Healers and Seers (called Ovates) - tended to wounds and illnesses and predicted the future -- determining best time to plant and harvest for example
Bards -- Celts did not write stories, they were passed along from one generation to the next orally through story-telling. Bards were also historians.
Druids -- philosophers and teachers and advisers to the rulers and judges
There are many, many books written on the role of the Druids in Celtic culture. It is believed that the stone circles in the UK were not built by Celts, though they used them (though on that topic there is some controversy). The Celts believed in many gods and goddesses -- theirs was a polytheistic religion.
However, the following web-sites can give you a good enough start:
http://ezinearticles.com/?History-of-the-Celtic-Druids&id=1477679
http://www.google.ca/search?q=Celtic+Druids&hl=en&safe=active&client=fi refox-a&hs=1ur&sa=G&rls=org.mozilla:en-US

fficial&channel=s&tbs=tl:1&tbo=u&ei=Yn2IS7z8C4 aXtgf_0_DJDw&oi=timeline_result&ct=title&resnum=11 &ved=0CC0Q5wIwCg
http://www.britannia.com/wonder/michell2.html
http://en.wikipedia.org/wiki/Druid
Some people are reviving Druidism, but really, we don't know how Druids really performed certain rites etc. Druidism today is a paganGanew/revived pagan) religion.
http://www.religioustolerance.org/druid.htm
http://www.celticholidays.ca/
I've been informally researching Celts and Druids since ... around early 1980s and I don't know about the religion either!
Druidism is NOT the same religion as Wicca (Witchcraft).