Demonology is the study of demons or beliefs about demons. They may be human, or nonhuman, separable souls, or discarnate spirits which have never inhabited a body. A sharp distinction is often drawn between these two classes, notably by the Melanesians, several African groups, and others. The Islamic jinn, for example, are not reducible to modified human souls. At the same time these classes are frequently conceived as producing identical results, e.g. diseases. (source)
DEVIL refers to Satan, also known occasionally as Abaddon, Beezlebub, and Diabolos. There is only one devil, and is generally described as having fallen from Heaven and rules over Hell.
DEMON refers to a supernatural being - originally the term didn't have negative connotations, but in modern times 'demon' has a negative connection with Hell and the devil. Demons appear in a lot of religions, sometimes as messengers, other times as tricksters, or malevolent beings who possess or torment humans. Demons are often associated with animals, fallen angels, and certains stars, colours or directions.
In the past, Demonologists translated ancient grimoires, analysed them, compiled them and discussed the theological implications and connections they had. While I consider myself a demonologist in training, my aim isn't to do those things. Instead, I aim to learn from their works, compile information about demons and demonology and curate a digital archive and directory for demon-related information.