"While the common hedge wizard or sorcerer gain their powers from a natural disposition towards magic, more civilized mages strive to understand the magical currents of the world. Theoretical magic is thus the field of understanding magic in a logical sense, often through the use of geometrical figures that represent different degrees of understanding.
In accordance with current belief the present theory explaining the structure of the world identifies five different planes of existence. These are known as the Mortal Realm, in which the world of men exists, the Spirit Realm, where spirits and other incorporeal creatures exists, the Realm of Unlife, where spirits deprived of their will to exist will journey, the Abyss, home plane to all demons, and finally the Elemental Planes, home to the elements of earth, fire, water and wind. Mages have identified that when a self-conscious being dies its soul will detach from the Mortal Realm and travel to the Spirit Realm. There it will stay until the spirit grows tired enough to forget much of the person it was in life and eventually what life was as well. In this stage it will drift from the Spirit Realm into the Realm of Unlife.
The fact that souls can travel between these places freely under these conditions have led many mages to attempt to travel to another realm using their powers, or have the realm come to them through a dimension portal. Needless to say such practices are dangerous but have given mages the possibility to understand and interact with the beings existing on the other planes."
The description above is taken from the introduction of the Magic Expansion to Centuria. In it you will find a detailed description of the four dimensions outside of the Mortal Realm and the Powers and creatures associated with them. You can find the Magic Expansion available on the Downloads page.