Nice description: "Generically referring to malevolent beings. Class One demons are the fallen Elder Gods. Class Two demons are devils and other lesser beings believed to have formed along with the gods after Atum's purge. Also includes gods that have degenerated. Class Three demons are extradimensional horrors."

See also:
http://en.wikipedia.org/wiki/Lemegeton
and:
http://en.wikipedia.org/wiki/Ceremonial_magic