In the hierarchy of demons, the lowest caste is composed of the demi-demons. Barring changes in station due to heredity, the half-mortal / half-demon offspring exist at the fringes of demonic society. They do, however, have a very important ability which makes them extremely useful. Their nature allows them to travel between the mortal realm and the various circles of Hell with impunity, without the restrictions full demons face. Likewise, their existence is partially outside the realms of time as understood by mortals, allowing them to appear in multiple locations at the same time, or to travel through time to a limited extent.
Although the term Nephilim is used as a term referring to all chosen demi-demons, this is not strictly accurate. There exists a myriad of types of demi-demons, as varied in type and ability as demons themselves. However, certain habits and characteristics of demons and humans have led to the majority of demi-demons being classed into four primary types.
The Nephilim (also known as "The Fallen Ones" or as "Progenitors of the Fall") are the largest group. The progeny of intelligent, free-willed, and sapient demons, the Nephilim are the most human in appearance. They are also as varied in power and talent as the demons and humans that conceived them. As a general rule, the more powerful the demon, the more powerful the Nephilim. This category of demi-demon is the most likely to concern themselves with the affairs of mortals, and the most likely to bring their demon parents to the mortal realm.
Secondly are the Emim. Known colloquially as "Giants," or more poetically as "The Dreaded Ones," Emim are Nephilim on a larger scale in either power or stature. Unable to move quietly and unnoticed in mortal societies, Emim either draw attention and acclaim to themselves or retreat to uninhabited areas. Most human "giants" have an Emim ancestor, and most of these ancestors were warriors of fearsome stature.
More disturbing than the previous two are the Rephaim, "The Shades of Death," "False-Life," the antitheses of nature. They exist in the state between the living and the dead, animating non-living flesh and giving it the semblance of life. The seed of new Rephaim is not well understood, but it is known to involve possession of a fetus in the second or third trimester. The Rephaim are not limited to a single body, and can be the most deadly of the demi-demons. Although individually weak and easy to destroy, sheer numbers can overwhelm where power would not.
The final and most abhorrent on our list is the Anakim. Known as "The Long-Necked," "The Serpentine," and most lingeringly to modern times as "Dragons." In the dimensions of Hell there exist great beasts that make up in ferocity what they lack in intelligence. A fairly common target of summoning spells, these beasts have more opportunity than most to escape their bonds and wreak havoc. An unshackled beast of this description invariably kills all males and mates all females around him until banished or killed. From such unions came the first Dragons, huge beasts that could think, reason, and desire as men do.
Adding more confusion to the classification of demons and their progeny is that the most powerful of the Lords of Hell have mutable shape, allowing them to create demi-demons of varying types that may or may not mirror this general classification system...
-Excerpt from "Demonology: A New Study" by Jon