(Typical Demons that are called as such in character are often the Dictionary of Mu definition. Both definitions are demons mechanically speaking, even if they are not called such.)

In Sorcerer demons may represent a variety of different entities and concepts depending on the campaign. They may be demons, aliens, abstract concepts, the sorcerer’s subconscious will, or dozens of other things. In Dictionary of Mu demons are the dead. Dead concepts, dead people, dead worlds, demons are everything that has come and died.

Once bound, the sorcerer must provide for her pet's specific needs, which may be straightforward (food) to bizarre (lick four different cat's eye marbles a day). In exchange, the demon provides its powers to the sorcerer. But no contract is perfect. The demon has its own desire and will. (Even if demons are supposed to be technological devices or whatnot, you still treat them as if they had their own motives, needs, and desires.) And whatever motivates a demon, surely it will not be kind to Humanity. The sorcerer is always in a balancing act, between conflicts with her demons, and conflicts with the larger world. The demon's powers are constructed from a simple effects based system. Just about any notion of "demon" can be sustained by the rules, from slithering pit fiends to alien, self-aware spaceships. It only requires a little creative interpretation.

Demons come in five basic flavors.

Passers Which look more or less like normal folks.
Inconspicuous Which are hidden most of the time.
Objects Your famous demon sword, or perhaps a ray gun.
Parasites These occupy a host's body and provide their powers through her. This is often the sorcerer herself.
Possessors These take over the host.