How does your publisher make money?
Traditional publishers make their money selling books - that's why they're so picky about what they will publish. Back in the day, vanity publishers made their money from writers paying to have their books printed. In the new market, print-on-demand has made it possible to publish a book yourself and sell it worldwide for the minimal cost of copyright registration and ISBN purchase.
Some writers aren't comfortable doing it all themselves, so there are "self-publishing" and "hybrid" companies that will help you get that book into print. They're making most of their money selling services to authors. Some of those companies are expensive but legitimate; others are a complete rip-off. Investigate any company you consider using for self-publishing.
To read about my personal experience hiring the wrong company, read Never Pay to Publish – and OMG. (The OMG's about a daring surf rescue where my daughter and her husband risked their own lives to save three young strangers.)
For a more indepth description of publishing options see Deciding How to Publish Your Book. It also includes links to watchdog lists that help you avoid current scams.
There's also the option of hiring a book shepherd - someone to walk you through the self-publishing process, helping you become your own publisher. Some companies act primarily as printers and distributors of your work - such as Amazon's Kindle Direct Publishing and Ingram's IngramSpark. They make money when your book sells.