There are several types or factions of demonology, which correspond to different religions. A few of the most recognizable being Christian Demonology, Jewish Demonology, and Occult Demonology. Most branches are connected by the fact that all believe in "evil", that "evil" just goes by different names. A few notable demons include Mephistopheles, Lucifer, Beezlebub, Iblis and Mara.
Christian demonology deals with the war between good and evil, or the war between God and Lucifer. Occult demonology is the type of demonology people tend to think of when they hear the word. This type deals with occultists trying to harness the "demons" to get them to do the occultist's bidding. Jewish demonology claims there are over seven million demons! This multitude of demons is then divided down further into various groups, based on what each demon does.
When trying to decipher Demonology, I have found that books called "demon dictionaries" have been quite helpful. They give a list of demons and which regions/religions/purposes they are related to. It really is an interesting subject.
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