Grab, The fourth book gets into it a lot more. The term mud-blood is used to denote a magic user with a muggle parent or two. I forget what the term is that is used for a wizard of wizard family who shows less than normal talent. Some of the tension and conflict in the story come from the factions within the wizard community over the issue of pure-bred vs non pure-bred wizards: can you guess where Voldemort might be on that one? One thing that I find interesting about the HP books is that J.K. Rowling has shown an ability to address issues that arise in one book in a subsequent book. I don't think they are great literature but they are fun and Rowling seems like an intelligent woman.