From the publisher's website:
Welcome to Ars Magica, and to the world of Mythic Europe. It is a place where the glories of the Classical world are dust and the promise of the Renaissance is yet to come. The time is the 13th century--1220 to be exact. Life is a struggle: wars and plagues stalk the land, the Church and kings rule with an iron fist. Yet Mythic Europe is also a place of magical wonder, inhabited by all the creatures of folktale and myth. What the folk of the land believe holds true: faeries and rural spirits must be placated; demons corrupt everything they touch; divine power is accessible through prayer; and magic is everywhere.
Against this background, you will play a magus, a member of the mythical Order of Hermes. You and your fellow players will also portray the loyal companions and grogs that stand with the magi in their covenants. These stalwart protectors provide a buffer between the magi and the mundane world that often misunderstands their power and motives. This is the setting for Ars Magica.