*In one interpretation, this means the offspring of angels and humans. That's not the only interpretation, though.
It also points out that Noah's generations were 'Pure' (Genesis 6:9) that being important for the continuation of the blood line of Christ. If you read the book of Enoch chapters 6 and 7 you find it fits perfectly in Genesis 6. In chapter 7 of the book of Enoch it talks about how they started to 'Sin' aginst the animals. I'm guessing a form of beastiality. Thus you get....animal people. But that there were Nephilim even after those days (Genesis 6:4). In Numbers 13:33 the nephilim were so big the people were in comparison, grasshoppers. If no funny breeding went on why is there super freaks? Goliath and his brothers were all Nephilim. (Numbers 13:33 talks about them running into giants)
You are right though, Lilith herself is a mystery. Judging from all the weird breeding I think they just stuck her in that catagory. She is known as the screech owl or harpy in some circles a ravager/rapist of men and all that is really just.....made up. I think where the two mix is the old Jewish mystic faith of Kabbalah. Jewish shamens speak of her in Zohar I believe....I don't remember though. I don't pay attention to Kabbalah it's not my thing. I had fun responding though I don't always get to blab about freaks.....or do I?
THere are two origins for Lilith: the first is that she was a rather generic 'night demon', almost universally described as having absolutely black hair, who variously seduced people, spread diseases, taught witchcraft, and killed children. The second, coming from Muslim tradition, is that she was Adam's first wife, made at the same time and co-equal with him, but she refused to submit to Adam and was cast out. Then the Almighty made Eve from Adam's rib.
I personally tend more to the former. According to my sources 'Lilit' translates roughly to 'ghost', 'night wind', 'owl' and 'night hag'.