Nephilim

Genesis Chapter 6, verses 1 through 4 mentions Nephilim:

"Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; nevertheless his days shall be one hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.
They are mentioned again in Numbers chapter 13, verses 32–33, in a description of the inhabitants of Hebron:

"So they gave out to the sons of Israel a bad report of the land which they had spied out, saying, "The land through which we have gone, in spying it out, is a land that devours its inhabitants; and all the people whom we saw in it are men of great size. There also we saw the Nephilim (the sons of Anak are part of the Nephilim); and we became like grasshoppers in our own sight, and so we were in their sight.
There are five common views regarding the identity of the Nephilim.

They were the hybrid offspring of fallen angels and human women.
They were the offspring of descendants of Seth with those of Cain.
Webster's 1913 Dictionary defines the word simply as "Giant." Thus, any especially tall, powerful, large, or mighty man would be described in ancient times as Nephilim. Nephilim may simply mean "giant," champion, or strong man.
The term might not refer to any specific race or group of people but is a label similar to "hero," a legendary figure, or "great man." The Bible itself describes the Nephilim as "Those were the mighty men who were of old, men of renown." In this view, the Bible tells us that Nephilim is a legendary or heroic figure, a mighty warrior, already renowned in legend by the time Moses wrote down Genesis. Using the Bible to define itself, Nephilim means a man of legend, one recounted in legendary tales.
They are not historical figures and are ancient imagery with questionable meaning.

http://www.nwcreati on.net/nephilim. html
The Nephilim were an antediluvian race (pre-flood) race which are referred to in the Bible as giants. They were reportedly the children born from the "daughters of men'', and the "Sons of God''. It is most important to note that they are mentioned almost simultaneous to God's statement that He would destroy the earth by flood, and it seems from this association that their affect upon mankind was one of the primary justifications that brought the destruction.

Genesis 6:1-7
1 When men began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of men were beautiful, and they married any of them they chose. 3 Then the LORD said, "My Spirit will not contend with man forever, for he is mortal ; his days will be a hundred and twenty years." 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. They were the heroes of old, men of renown. 5 The LORD saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. 6 The LORD was grieved that he had made man on the earth, and his heart was filled with pain. 7 So the LORD said, "I will wipe mankind, whom I have created, from the face of the earth—men and animals, and creatures that move along the ground, and birds of the air—for I am grieved that I have made them." NIV

The Sons of God

Nobody knows what the ''Sons of God'' were, but the most obvious interpretation is that the Nephilim were a hybrid race between two distinct but sexually compatible beings. Many believe the ''Sons of God'' were fallen angels who corrupted mankind. The ''Sons of God''' are clearly distinguished from the daughters of men. This suggests the ''Sons of God'' were either not born on earth or simply righteous men as opposed to earthly. We can only speculate, but as angels are not biological organisms, they are not likely to be reproductively compatible with humans.

Another logical interpretation for the ''Sons of God'' is they were other created men. It is largely assumed that Adam and Eve were the only humans created in the beginning, but the Bible does not describe every person on earth, nor even everyone that was important to God. Only key individuals or situations are included within the text, and we can not say with certainty that God only created one pair of humans.

The phrase "sons of God" used in Genesis 6 is “bene elohim”, which used apparently only three other times in Scripture and in each of the other cases it is found in the book of Job which is accepted as one of the most ancient books written. Job 1:6; 2:1; 38:7.

Job 1:6 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
Job 2:1 Again there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them to present himself before the LORD.
Job 38:7 When the morning stars sang together, and all the sons of God shouted for joy?

And Also Afterward

In Genesis 6:4 it is said the Nephilim were on the Earth in those days - and also afterward. This seems to clearly imply that the Nephilim were also on the Earth after the flood of Noah. Although the Nephilim are only mentioned one other time in scripture after Genesis 6, during a false report brought back to Moses by the explorers of Canaan, this reference would suggest that the Israelites were familiar with the Nephilim.

Numbers 13:30-33
30 Then Caleb silenced the people before Moses and said, "We should go up and take possession of the land, for we can certainly do it." 31 But the men who had gone up with him said, "We can't attack those people; they are stronger than we are." 32 And they spread among the Israelites a bad report about the land they had explored. They said, "The land we explored devours those living in it. All the people we saw there are of great size. 33 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them." NIV

http://www.pantheon .org/articles/ n/nephilim. html

Genesis 6:4 states "The Nephilim were on the earth in those days --and also afterwards-- when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown." The Nephilim were a race of giants that were produced by the sexual union of the sons of God (presumably fallen angels) and the daughters of men. Translated from the Hebrew texts, "Nephilim" means "fallen ones." They were renowned for their strength, prowess, and a great capacity for sinfulness.
The origination of the Nephilim begins with a story of the fallen angels. Shemhazai, an angel of high rank, led a sect of angels in a descent to earth to instruct humans in righteousness. The tutelage went on for a few centuries, but soon the angels pined for the human females. After lusting, the fallen angels instructed the women in magic and conjuring, mated with them, and produced hybrid offspring: the Nephilim.

The Nephilim were gigantic in stature. Their strength was prodigious and their appetites immense. Upon devouring all of humankind's resources, they began to consume humans themselves. The Nephilim attacked and oppressed humans and were the cause of massive destruction on the earth.

Two texts of central import to the story of the Nephilim, the Bible and the Dead Sea Scrolls, mention several names for the Nephilim. The diverse kinds of these giants are cited in several passages. They are variously referred to as Emim, or "Terrors" (Gen. 14:5; Deut. 2:10), Rephaim, or "Weakeners" or "Dead Ones" (2 Sam. 23:13; 1 Chron. 11:15), Gibborim, or "Giant Heroes" (Job 16:4), Zamzummim, or "Achievers" (Deut. 2:10), Anakim, or "Long-necked" (Deut. 2:10; Josh. 11:22, 14:15), and Awwim or "Devastators" and "Serpents." Other giants are mentioned in these texts as well, such as Goliath (2 Sam. 21:19), a giant with twelve fingers and twelve toes who is mentioned as one of the Rephaim (2 Sam. 21:20), and a tall Egyptian (1 Chron. 11:23). The passage of Numbers 13:26-33 recounts the Nephilim of Canaan that Joshua and the other Hebrew spies saw. Furthermore, according to Judaic lore, a certain one of the Nephilim, Arba, built a city, Kiriath Arba, which was named for its builder and is now known as Hebron.

The wickedness of the Nephilim carried with it a heavy toll. Genesis 6:5 alludes to the corruption that the Nephilim had caused amongst humans and themselves: "The Lord saw how great man's wickedness on the earth had become..." Their evil rebellion had incurred both the wrath and grief of God. God instructed the angel Gabriel to ignite a civil war among the Nephilim. He also chose Enoch, a righteous man, to inform the fallen angels of the judgment pronounced on them and their children. God did not allow the fallen angels any peace, for they could not lift their eyes to heaven and were later to be chained. The end of the Nephilim came about in the war incited by Gabriel, in which the giants eventually annihilated each other.