According to the New American Bible, the Nephilim appear as part of the "increasing wickedness of mankind". Their mention does account for the "giants" of Canaan, whom the Israelites also called the Nephilim, since according to Genesis 6:4 "The Nephilim were on the earth in those days--and also afterward--when the sons of God went to the daughters of men and had children by them." (NIV translation). This verse, which precedes and introduces the passage of the Great Flood thus implies that the Nephilim actually came to the earth at least twice. The first of which were destroyed in the flood, and the second succeeding the flood. Thus it is possible the "giants of Canaan" were the direct result of the Nephilim. The reference introduces the story of the flood with a moral orientation.
Also, the commentary suggests that the phrase "(as well as later)" stated above is a reference to the Book of Numbers 13:33, how the Israelites likened the tall aborigines ("Anakim") to the Nephilim, possibly due to seeing the very tall structures of Canaan that appeared to have been built by a race of giants.
The New American Bible commentary draws a parallel to the Letter of Jude and the statements set forth in Genesis, suggesting that the Epistle refers implicitly to the paternity of Nephilim as heavenly beings who came to earth and had sexual intercourse with women.
The angels too, who did not keep to their own domain but deserted their proper dwelling, he has kept in eternal chains, in gloom, for the judgement of the great day. Likewise, Sodom, Gomorrah, and the surrounding towns, which, in the same manner as they, indulged in sexual promiscuity and practiced unnatural vice, serve as an example by undergoing a punishment of eternal fire.
According to the New Oxford Annotated Bible, the Genesis verses only indicate that the Nephilim existed at the same time as the products of divine-human intercourse.
The view that the reference is to descendants of Seth and Cain, is in fact earlier than the fourth century; for example in the writings of Sextus Julius Africanus, as well as throughout the Clementine literature.
The Targum Yonathan states that they were given this name because they were descended from fallen angels.
The Nephilim come from a union between Sons of God (בני האלהים "b’nei ha-'elohim" Lit. "Sons of the powers" ) and "daughters of man".
In Aramaic culture, the term Nephila specifically referred to the constellation of Orion, and thus Nephilim to Orion's semi-divine descendants (cf. Anakim from Anak); the implication being that this also is the origin of the Biblical Nephilim. Some commentators have suggested that the Nephilim were believed to have been fathered by members of a proto-Hebrew pantheon and are a brief glimpse of early Hebrew religion, most of the details of which were later edited out from the Torah (or at least would have been edited out when, as some claim, it was redacted together), and that this passage may have offered monotheistic Hebrews a way to fit semi-divine pagan heroes into their cosmogony.
In the Hebrew Bible, there are a number of other words that, like "Nephilim", are sometimes translated as "giants":
"Rephaim" (or Rephaites) is a general title that the Book of Joshua states was given to the aborigines who were afterwards conquered and dispossessed by the Canaanite tribes. The text states that a few Rephaim had survived, one of them being Og, the king of Bashan. Og of Bashan is recorded as having a 13-ft long bed.
Only Og king of Bashan was left of the remnant of the Rephaites. His bed was made of iron and was more than thirteen feet long and six feet wide. It is still in Rabbah of the Ammonites.
The Rephaim may have been the same Canaanite group known to the Moabites as Emim, i.e., fearful, and to the Ammonites as Zamzummim. The second of the Books of Samuel states that some of them found refuge among the Philistines, and were still existing in the days of David. Nothing is known of their origin, nor of anything specifically connecting them with Nephilim, though the connection is made by Jewish tradition.
Anakim (or Anakites) are the descendants of Anak, and dwelt in the south of Canaan, in the neighbourhood of Hebron. In the days of Abraham, they inhabited the region afterwards known as Edom and Moab, east of the Jordan river. They are mentioned during the report of the spies about the inhabitants of the land of Canaan. The book of Joshua states that Joshua finally expelled them from the land, excepting a remnant that found a refuge in the cities of Gaza, Gath, and Ashdod. The Philistine giant Goliath, whom David later encountered, was supposedly a descendant of the Anakim.
The land, through which we have gone to spy it out, is a land that devours its inhabitants, and all the people that we saw in it are of great height. And there we saw the Nephilim (the sons of Anak, who come from the Nephilim), and we seemed to ourselves like grasshoppers, and so we seemed to them.
It is more commonly suggested by traditional Jewish sources (such as the Midrash) that the spies saw large and powerful inhabitants in Canaan and because of their own fears, cowardice, and inadequate faith in Yahweh, saw themselves as grasshoppers in the eyes of the Canaanites, whether they were actual 'giants' or not.
The story of the Nephilim is chronicled more fully in the Book of Enoch (part of Ethiopian biblical canon). Enoch, as well as Jubilees, connects the origin of the Nephilim with the fallen angels, and in particular with the Grigori (watchers). Samyaza, an angel of high rank, is described as leading a rebel sect of angels in a descent to earth to have sexual intercourse with human females:
And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. And the angels, the children of the heaven, saw and lusted after them, and said to one another: 'Come, let us choose us wives from among the children of men and beget us children.' And Semjaza, who was their leader, said unto them: 'I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin.' And they all answered him and said: 'Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.' Then sware they all together and bound themselves by mutual imprecations upon it. And they were in all two hundred; who descended in the days of Jared on the summit of Mount Hermon, and they called it Mount Hermon, because they had sworn and bound themselves by mutual imprecations upon it...
According to these texts, the fallen angels who begat the Nephilim were cast into Tartarus/Gehenna, a place of 'total darkness'. However, Jubilees also states that God granted ten percent of the disembodied spirits of the Nephilim to remain after the flood, as demons, to try to lead the human race astray (through idolatry, the occult, etc.) until the final Judgment.
In addition to Enoch, the Book of Jubilees (7:21-25) also states that ridding the Earth of these Nephilim was one of God's purposes for flooding the Earth in Noah's time. The Biblical reference to Noah being "perfect in his generations" may have referred to his having a clean, Nephilim-free bloodline, although it may be inferred that there was more diversity among his three daughters-in law.
Some individuals and groups, including the Ethiopian Orthodox Church, St. Augustine, John Calvin, and the Latter-day Saints, take the view of Genesis 6:2 that the "Angels" who fathered the Nephilim referred to certain human males from the lineage of Seth, who were called sons of God probably in reference to their being formerly in a covenantal relationship with Yahweh (cf. Deuteronomy 14:1; 32:5); according to these sources, these men had begun to pursue bodily interests, and so took wives of the daughters of men, i.e., those who were descended from Cain. Not only is this unequivocally stated in Ethiopian Orthodox versions of I Enoch and Jubilees, but this is also the view presented in a few extra-Biblical, yet ancient works, particularly the Conflict of Adam and Eve with Satan. In these sources, these offspring of Seth were said to have disobeyed God, by breeding with the Cainites and producing wicked children "who were all unlike", thus angering God into bringing about the Deluge.
Nowhere is the Ethiopian view presented more explicitly than in the Conflict of Adam Book 3, chap. 4: