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."
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.
Here we apparently see the remnants of an earlier Hebrew religion. We don't know who the "sons of God" are. Of course in the New Testament there is only one, but here there are many. They cross-breed with humans and give rise to race called the Nephilim (translated as "giants" in some versions but left intact here in the New International Version). Evidently this hybrid race was wiped out in the flood (soon to come, so get the popcorn popper heating), because we never hear from it again. If you are interested in this question (which I am not, particularly) you can read more about it in Wikipedia, which tells us that Nephilim may be derived from an earlier Aramaic religion and refers to the sons of Orion, the celestial hunter.
In Jewish tradition, and mainstream Christian tradition, the "sons of God" are angels. Now, if you're like me you're finding this all quite grotesque and even rather offensive -- not to mention irreconcilable with the New Testament and modern monotheistic ideology.
While it's progress that God decides to cut the human life span down from 900+ years to 120, obviously we still haven't made it to reality. Later, of course, the Bible tells us that our days are three score and ten, which was about the max in those days, although nowadays most of us hope for just a few years more. His reason for reducing the life span seems to be that he gets tired of us after 120 years. So, once again, he decides that the prototype was flawed. In this case, he decides to do some redesign work rather than just kicking our butts. But, as we shall see, this only a brief reprieve.
What a putz.