The Bible mentions the Nephilim twice:
Gen. 6:4 The Nephilima were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
Num. 13:33 We saw the Nephilim there (the descendants of Anakb come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them.”
The bottom line is we’re not sure who the Nephilim are. Some conjecture that they are some sort of super human race, based on the phrase, “sons of God went to daughters of humans” but most likely, they were an unusually tall regional population.
Rather than rehash the discussion, I’d refer you to this excellent site that covers the 3 most popular understandings of this passage. Click here.