Okay, I'm going to do my best to explain demonology, but keep in mind, this is a very complex topic.  I'm going to do my best to keep it as simple as possible, but in so doing, realize that means I'm speaking in HUGE generalities.  This means, when I use the words, most or the like, I am not speaking for ALL forms of demonology.

Demonology exists in many different forms under many differing names, depending on the faith tradition that a person comes from.  Demonology can be seen in Roman Catholism, other forms of Christianity, Islam, Hinduism,  Judaism, Zorastrianism, Buddhism, and many other faith systems.  I do not intend to go into all of their individual beliefs on demons or demonology.  Just keep in mind, apart from names and semantic differences, demons and demonology shows up in MANY different religions.

First off,  I'm going to list some commonalities (faith tradition excluded) in varying forms of demonology. Demons, in most traditions, refer to negative, malicious, non-corporeal (without physical form) entities.  Depending on the tradition, they are seen as fallen angles, servants of Satan, denizens of hell, manifestations of evil, adversaries of God, etc. (Note:  originally the word demon from the Greek daemon, daimon, or dæmon, referred to intermediary spirits/beings that existed between the level of humanity and the Gods and did not necessarily refer evil beings.  There are forms of current demonology that still hold this definition and not the one above, mine included.)

Another similarity between most types of demonology is the recognition of some sort of hierarchy that is, the understanding of the relationships between these entities and the structure and organization interrelating them to each other and the world/universe outside of them.

From there, I further delineate and separate demonology into two basic categories (according to me).  The two categories are scholarly demonology and classical demonology.

Scholarly demonology is purely the study of demons; their hierarchies, forms, relationships etc.  There are many books written from various religious perspectives in which people have categorized demons and their forms.  In a way, scholarly demonology is very similar to biological classification.  Scholarly demonologists take a demon, find its characteristics, and place it into its category depending on the characteristics that the scholar has previously defined.  When I refer to scholarly demonology, I'm talking of learning and study, with little or no personal involvement with the demons themselves.

Now, classical demonology, although it involves the knowledge of scholarly demonology, in general is a much more hands-on form of demonology.  The classical demonologist is the person who has not only learned the nature of demons, but they have also studied in order to learn ways to control them.  When you hear of someone performing an exorcism, they are what I refer to as the classical demonologist.  The classical demonologist has the knowledge and training to be able to expel demons from people/things/places etc.

Classical demonologists come in many different forms, depending on the faith tradition that they come from.   The faith tradition dictates the type of training, the rituals (if any), and ways in which these entities are dealt with and viewed.   Some traditions view demons as beings that need to removed from people and places, in order to restore goodness and peace to those people/places. However, other traditions of demonology see these entities as things to summon and control in order to gain favors, knowledge, power, etc.

Well, I'm sure this post has been long and headache inducing enough.  I realize that I have just skimmed the surface of the topic, so to speak, and hope I answered your question to some degree.  Its a very in depth topic and has many differing views and definitions depending on faith tradition and demonology type practiced.

Below are some links to more general information on demonology if anyone is interested.  (Articles are from various religious and historical perspectives.)

Article written by me originally in a post to Southern Michigan Paranormal Investigators