de·mon·ol·o·gy (dm-nl-j)
n. 

1. The study of demons.

2. Belief in or worship of demons.

Rather a simple definition for an art that is as varied as there are opinions on who, or what these entities are, why they are feared by some, and eagerly pursued by others. 

 I will state before anything else that this is based from my opinions and experiences in this fascinating realm of magic, any "fact" when dealing with these creatures and their arts are rarely that. 

With that disclaimer out of the way, let us get on to the root of this particular practice.

The demons...

It is well known that the term demon is actually derived from the greek word deamon, however, the greeks did not see these creatures as being the evil beings portrayed by the common era but a catch all phrase for supernatural beings that are neither gods or mortal but  inferior divinities and ghosts. 

So when did they become perceived as evil creatures of "dark" magicks, corrupting the innocent and destroying all the good in the world?

It started with Judaism from what I can find and it's child christianity that created them into the wicked fiends of western culture and contemporary religious belief since the first century of the common era. 

It apparantly started with the  The Septuagint (LXX) translation of the Hebrew bible; from which the Old Testament is derived, the New Testament and other works of western medieval culture such as the Ars Goetia and a variety of  grimoires written between the late medieval period an the eigtheenth century that have them as the well known sources for ceremonial magicians, various priests and others who follow those texts to banish and summon at will.. 

It is during these times that they were taken from being worshipped as gods by grand cultures such as the Babylonians and Assyrians; and being made into rebel angels against God and all his creations, cast down from heaven for refusing subservance to those created in God's image.  This is most nobely demonstrated in the  Koran which acknowledges Azazel's reply when asked to bow before Adam;  "Why should the son of fire fall down before a son of clay?"

So we now come to the antiheroes glamorized by classic authors such as Milton and Marlowe, exalted to hero and villian status by comic book companies and used for bed time entertainment and late night horror movies by popular media. 

So now we know the traditional meaning of who they are, lets look at the theories of what they are away from well known dogma.

Some theorize that they are not actual entities, but simply splices of the self, sort of the ids of our psyche emitted as external forces to pass blame to for that which is inherent to the more animal side of human nature, but popular society consideres evil or dark by the more religious kind of people..

There are  those who believe and worship them as they were before the christian faith altered their reasons for existance, simply using the term demon or devil to discuss supernatural entities that for whatever reason are not happy with the mortals of our world and react to us in a variety of ways, some people have more pleasurable experiences while others seem to be terrorized and tortured by these creatures of the shadowy corners of our dreams...

Now let's look at some magickal practices.

the most common use is for or against other demonic entities.

A practicioner can use a particular demonic entity for a need they may have, assistance in achieving something in this world that the more typical visualization and focus may not be quite enough to achieve.  The goetia is a very good resource for finding the many uses that established entities may have.  There are also numerous texts on the proper circles and triangles, chants and rituals to summon these creatures under the magicians control.

There is also a more spiritual approach, rather than commanding them and binding them to the magician, it's more akin to contacting them as one would contact any spiritual energy, inviting them in with proper protections and preparations of course.

And there are always those messing where they shouldn't be and calling forth something they can't control or yet lack the ability to control.  Tormented and tortured through dreams, thoughts and even physical attacks. 

For these beginner mistakes there are always a variety of banishings, the most common is the lesser banning ritual of the pentagram(lbrp).  Again it's from a ceremonial magicians viewpoint on dealing with them.  Calling upon archangels to counter the fallen angels influence.

A recent publication offers a more pleasant usage of communicating and even spiritually copulating with these entities.  Donald Tyson's Sexual Alchemy goes into great detail, along with some examples of those who've been willingly or unwilingly seduced by these entities, almost always starting with sexually frustrated or those who are inhibiting their needs and often times suffering for the faith of their god.

To sum it up, any search on the Golden dawn, Goetia,  or other ceremonial magick publication can go into much more detail on any other questions or curiousities you may develop.

However I'll leave you with some wisdom and a warning by one of the Roman Catholic church's original experts on the world of demonology, Sir Albert Magnus.

 " A daemonibus docetur, de daemonibus docet, et ad daemones ducit"

(It is taught by the demons, it teaches about the demons, and it leads to the demons).