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

When men began to increase in number on the earth and daughters were
born to them, the sons of God saw that the daughters of men were
beautiful, and they married any of them they chose. 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." 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. 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. The LORD was grieved that he had made man on the
earth, and his heart was filled with pain. 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."

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

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." But the men
who had gone up with him said, "We can't attack those people; they are
stronger than we are." 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. 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."
What Are Nephilim?