What are The Nephilim?

A friend of mine mentioned these vreatures called the Nephilim. I asked him what they were, and he explained; "In the Old Testament, it was once belived that fallen angels would come down to Earth and reproduce with human women.... their offspring grew up to be giants, called the Nephilim".
He also said that one of the reasons why God caused the great flood with Noah's arc was to be rid of the Nephilim. I don't remember the Bible ever mentioning anything about these creatures. Any insight? Does anyone know about other strange creatures mentioned in the Bible like these? Tweet