Merlin, of King Arthurian legend, is probably the most famous druid. 

Druids, however, acted as priests, teachers, and judges, since they were among the most learned Celts. 

Although the druids worshipped many lesser gods, they worshipped a supreme god they called Be' al that was symbolized by fire.  They also believe that individuals were immortal, because upon death their soul would be passed on to a newborn child, so basically they believed in reincarnation.    

Julius Caesar was one of the first to write about the druids when he was in Gaul, and it's from the Romans that we've gained most of our knowledge about the druids.