There are many worlds out there, and most humans are blind to them. These other planes have magical power in their very core, and the creatures of those worlds can use that magic to cast spells. Only a few humans have powerful enough souls to draw from that magic.

You are one such human. You, John Doe, are a User. You were born to a clan of magic users. From an early age you knew the truth of the world and you knew how to draw the magic from the other worlds to use for your own purpose.

While Users can draw power from many planes of existence, most mages usually focus on one plane. It's sort of their signature spell type.