Druidism is the umbrella-term used to describe the religion of the "Celtic" people, and specifically the Priest-Judge Caste of the "Druids".  Because "druidism" was an oral tradition, we have no written materials from the Celts themselves.

What we know of the religion of the Celts we have learnt from a variety of sources. The main problems we have is in interpreting these sources, especially in seperating the facts from the classical and later literary inventions. Archaeological remains show us solid material objects, and although open to personal interpretation votive offerings, burials and inscriptions give us a physical "feel" of  the people.

The Classical Writers, though often with personal agendas, provide us with knowledge of "Celtic" peoples. From "Classical Sources", the main references to Druids is in regards to "sacrificial ceremonies". Caesar, Tacitus and Strabo all refer to human sacrifice. Pliny refers to a Druidic ritual in which a Druid cuts Mistletoe before two white bulls are sacrificed. The problem with interpreting the Classical material is that its authors are not a part of the culture that they are describing, much in the way "Western Culture" has interpreted African or Native American Cultures. Often the dominant culture views them as childlike warriors or as the noble savage, merely caricatures images of a culture.

The Insular Writers, whether Irish Monks, Gildas, Giraldus or whoever, are often heavily influenced by "Christian" thought.  Most of what we refer to as "Celtic" comes down to us from this period.

The Classical Sources portray the Druids as either great Philosophers, seekers of Knowledge, keepers of Tradition OR blood-thirsty priests, sacrificing, burning, killing in the name of their pagan gods. The Insular Writers, despite their obvious Christianity, seem on the whole rather sympathetic to the Druids. Cathbad, the chief Druid from `the Tain` is a wonderful figure, as is the Druidess Fidelma, also from "the Tain". Though the tales have obviously been embellished they seem to be as close as we can get to an acurate description of a living Druid.

According to the texts, they were held in high regard, their wisdom was sought by Kings. We know that they performed rituals and shaman-like acts of magic. We know that they met in nemetons, places held sacred. We know that there were both male and female Druids. We know that the training to become a Druid was very extensive.