There are two forms of devil that exist: greater devils and lesser devils.
Greater devils are those that existed at the time of the War in Heaven, beings who were once angels who took the side of Satan against Yahweh. Greater devils are immortal, and their number is known. It is written in the Clavicua Salomonis Regis, that among the greater devils there are 72 arch-devils; and that below these are 6 legions of greater devils, commanded by the 46th Spirit Bifrons; these are composed of 66 cohorts, made of 666 companies within which are 6,666 greater devils.
Lesser devils were born of fallen angels who were sent by Yahweh to watch over the intelligent beings of the Earth, as told in the Book of Enoch [placeholder]. Enoch was the son of Cain, the son of Adam and Eve, and Cain's wife Lilim, who was the daughter of Lilith. Enoch called the angels who came to Earth "the Watchers." [placeholder] Because Enoch lived circa 2764 BC, (see Ussher) the presence of the Watchers far predates the occurrence of the War in Heaven.
Among these watchers were those who lusted for the beauty of women and began to procreate among them. The offspring of these unions were the Nephilim, a name that has been variously misunderstood by different writers. Among the Nephelim were some of the giants that are known to the world, notably the fire and stone giants (frost giants, hill giants, cloud giants, storm giants and cyclops have other origins). These were birthed from the children of Seth, the son of Adam. The Nephelim that were birthed from the children of Cain, however, and therefore of Lilith, became the lesser devils: bone and barbed devils, and more commonly the lemure, who form the greatest number of lsser devils.
Those watchers who gave in to their lust were called "the Fallen" long before the War in Heaven was fought. They did not fight in the War, but were welcomed into the Lower Planes after the War's end. Those angels who kept their passion in check are called the Grigori [placeholder], and still walk the Earth.
Lesser devils are not immortal; they live a life of no longer than 120 years and are therefore treated by greater devils as beings to be employed outside the realms of Hell. However, devils as a whole rarely leave their planes of existence, preferring to command demons to hazard the Prime Material Plane. If the actions of demons becomes suspect to the Gods, a lesser devil will be sent to the Prime Material to investigate and to ensure obedience. All the cohorts and companies of Bifrons are composed of lesser devils. Greater devils do not appear on the Prime Material unless they are deliberately called.