The Nephilim (plural) are the offspring of the "sons of God" and the "daughters of men" in Genesis 6:4, or giants who inhabit Canaan in Numbers 13:33. A similar word with different vowel-sounds is used in Ezekiel 32:27 to refer to dead Philistine warriors.
There are effectively two views regarding the identity of the Nephilim, which follow on from alternative views about the identity of the sons of God:
Offspring of Seth — The Qumran (Dead Sea Scroll) fragment 4Q417 (4QInstruction) contains the earliest known reference to the phrase "children of Seth", stating that God has condemned them for their rebellion. (Nonetheless, a few commentators dispute the interpretation of this reference.) Other early references to the offspring of Seth rebelling from God and mingling with the daughters of Cain, are found in rabbi Shimon bar Yochai, Augustine of Hippo, Julius Africanus, and the Letters attributed to St. Clement. It is also the view expressed in the modern canonical Amharic Ethiopian Orthodox Bible.
Offspring of angels — A number of early sources refer to the "sons of heaven" as "Angels". The earliest such references seem to be in the Dead Sea Scrolls, the Greek, and Aramaic Enochic literature, and in certain Ge'ez manuscripts of 1 Enoch (mss A-Q) and Jubilees used by western scholars in modern editions of the Old Testament Pseudepigrapha.However, "Angels" in this context has sometimes been considered to be a sarcastic epithet for the offspring of Seth who rebelled (see above). The earliest statement in a secondary commentary explicitly interpreting this to mean that angelic beings mated with humans, can be traced to the rabbinical Targum Pseudo-Jonathan, and it has since become especially commonplace in modern-day Christian commentaries.
Others do not take either view, and believe that they are not historical figures but are ancient imagery with questionable meaning. The fallen angels interpretation
The New American Bible commentary draws a parallel to the Epistle 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 footnotes of the Jerusalem Bible suggest that the Biblical author intended the Nephilim to be an "anecdote of a superhuman race".Genesis 6:4 implies that the Nephilim have inhabited the earth in at least two different time periods—in antediluvian times "and afterward." If the Nephilim were supernatural beings themselves, or at least the progeny of supernatural beings, there is a theory that the "giants of Canaan" in Book of Numbers 13:33 were the direct descendants of the antediluvian Nephilim, or were fathered by the same supernatural parents.
Evidence in favour of the "fallen angels" interpretation includes the fact that the phrase "the sons of God" (Hebrew, בְּנֵי הָֽאֱלֹהִים; literally "sons of the god") is used just two times outside of Genesis chapter 6. In both instances (namely, Job 1:6 and Job 2:1) the phrase explicitly references angels. This is supported by the Septuagint version of the Codex Alexandrinus, which, in Genesis 6:2, renders the phrase "angels of God". (more on wikipedia)