The nephilim were the offspring of a mortal woman and a seraph (plural: seraphim). The angels who came from the heavens taught man of magic and vanity. These angels had off spring with the mortal women known as the Nephilim.

When God wiped out the Seraphim and the Nephilim with a flood he trapped the Nephilim on earth because they were not completely of the heavens, so they could not enter heaven. Nor were they completely of the earth, so they could not die.

The Nephilim were the first to be called what is known as demons or djinn, forever trapped on earth without corporeal form. Nephilim inhabited the bodies of mortals in a form of possession which was believed could be passed by touch such as with the major motion picture Fallen. Keep in mind, biblically speaking, Azazel was actually the leader of the seraphim who came down from heaven to be with man, and NOT a Nephilim. After the flood, the seraphim were imprisoned in hell or buried alive deep, deep within the earth where they supposedly still reside.

Throughout the bible, the nephilim are typically refered to as giants (They inherited height from their angel lineage, but they did not inherit the wings).

A good reference point for the Seraphim and the Nephilim is the apocrypha which was once part of the bible but later removed due to questions over origin. The Greek Orthodox still retain the apocrypha within their version of the bible.