From Satan Service
Through time the term 'demon' has come 'round to dualistic notions as employed by the restrictive and judgemental to mean 'evil spirit', though in its Greek origins (daimon, daemon) it was variable of character and not unlike any other personal species ('human', 'djinn', 'mammal', etc.).
'Demonology' is the study of (typically specific) demons and negative spiritual hierarchies conceived by these dualistic religious.
'Demon summoning' is usually facilitated by a particular guidebook or directory (sometimes called 'grimoires' after a french Catholic standard) listing the presumed inhabitants of their locale.
By convention demons are composed of myriad species' body portions, may have reptilian or draconic elements, and will often feature faces on various parts of their anatomy, particularly the genitalia, anus, stomach, or chest. They may vary in colour and will quite often also sport predatory teeth and eyes, giving a fearsome and dangerous appearance.