The main passage on the Nephilim is Genesis 6:1-4. The biblical Hebrew term for “sons of God” in verse 2 of this passage is benei elohim. It only refers to angels, whether good or bad. The phrase “daughters of men” refers to human women. Hence, this passage is dealing with the fact that fallen angels intermarried with human women. The result was a race of Nephilim. Although often translated as the English word “giants,” the Hebrew word nefilim simply means “fallen ones.” They were the product of the sons of God and human women that had to be destroyed by the flood.
The angels who caused the sin are now confined in Tartarus, never to be freed to do this again. The only time they were mentioned again is in Numbers 13-14. The context is the story of the spies who were lying about what they saw in the land. While they claimed to have seen Nephilim, it was a false statement on their part. When Joshua conquered the land, he never ran into any Nephilim.
For further information on this important topic, we recommend Volume 5 of our Come and See series, titled A Study of the Angelic Realm: Angelology, Satanology, and Demonology.