POD is a technology. It's a way to print books. It's quite useful for printing small quantities, particularly if there is intermittent demand. LOTS of publishers who are not vanity houses or scam mills use POD technology. University presses spring to mind, as do very small limited runs of very tightly focused books. POD is not evil.
Vanity presses can use POD technology OR they can use webfeed technology. Vanity presses are essentially printers with some support staff. They'll help you print up nice editions of whatever you want. You pay for this. It's called vanity because they don't acquire the book. Acquire means there is an editorial staff choosing particular books to publish. Vanity houses do not maintain lists, issue catalogs or sell books in bookstores. Vanity presses are not evil
Self publishers can use POD technology or webfeed technology. Self publishers are not vanity presses in the everyday sense of the word. They are "vanity" in the sense that there isn't an acquisition but the two phrases are used to mean different things in publishing. Lots of people self publish for a lot of reasons. Self publishing is not evil.
POD/scam mills are companies set up to persuade you, the author, that printing your book with their company is the equivalent to having it acquired by a publisher. They charge you money. Unlike a respectable vanity press, they don't copy edit or produce high quality products. They are out to make money on volume. They prey on author's insecurities and lack of knowledge. POD/scam mills are the scum of the earth.
Whether a company is the scum of the earth depends on how they run their business, not how they print their books.
There are several POD companies that do not try to persuade you that you have but to print up books with them to be on your way to fame and glory. Lulu and CafePress come to mind. There are others I'm sure.
There will be a quiz.