Genesis 6:1-4 reads: “And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose… There were nephilim in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.”
Nephilim is often translated as “giants”, a legitimate and appropriate interpretation, but one which may be only partially accurate. A better definition might be “those who came down”, “those who descended”, or “those who were cast down.” The Anunnaki of ancient Sumerian texts is similarly defined as “those who from heaven to earth came”. Sitchin, Gardner , and Bramley have all identified the Nephilim as the Anunnaki, more specifically, essentially the rank and file.
The Anunnaki have also been equated with the “Watchers” (who are also mentioned in the books of Daniel and Jubilees), i.e. “Behold a watcher and an holy one came down from heaven.” — Daniel 4:13
According to Zecharia Sitchin and his interpretation of ancient Sumerian texts, the Anunnaki were extraterrestrials (aka “angels”?), who were an extremely long-lived race, potentially living as long as 500,000 years. Laurence Gardner  reduces this to more on the order of 50,000 years, and notes specifically that the Anunnaki were not immortal. He point out that no records are currently extant which relates to their natural deaths, but the violent deaths of Apsu, Tiamat, Mummu, and Dumu-zi are provided in some detail. (Sitchin and Gardner also disagree on the date of the Great Deluge/Flood; Sitchin assuming a time frame of 11,000 B.C.E., while Gardner assumes one of 4,000 B.C.E.)
Sitchin’s book, The 12th Planet, published in 1976 was the first modern volume to begin to describe the Anunnaki, their arrival on Earth supposedly some 485,000 years ago, and from where they had come — a planet called Nibiru. Sitchin believes Nibiru to be in an orbit about our sun, but in a strongly elliptical orbit which requires 3,600 Earth years to make a complete orbit. Nibiru’s perihelion (closest point of approach to the Sun) is thought to be within the main asteroid belt between Mars and Jupiter, at a distance from the Sun of approximately 2.75 A.U. (an A.U. being the distance from the Sun to the Earth). (the Annals of Earth include a detailed description of how Nibiru created the asteroid belt by destroying a planet, Tiamat, in roughly the same orbit, and which created the Earth in the aftermath, the Earth being a remnant of the greater, destroyed planet.)