The Celts were a loosely affiliated group ancient warrior clans and tribes that spanned an area from modern-day Turkey to Ireland. The word 'celt' comes from the Greek 'Keltoi.'
Contrary to popular belief, there is no such thing as a pan-Celtic culture or religion. Considering there is a span of over 10,000 miles between Turkey and Ireland, there is no way that a group of people that large could have any standardized way of life.
The Celts didn't write things down. Most of what we know about life in Gaul (Modern day France) and Britain is from records written by the Romans and the Greeks. The Romans sought to syncretize the British gods with their own as a means of conversion into the life ways of the average roman. This lead to the developement of the well documented Romano-Celtic period and religious practices.