Science Fiction and Fantasy Writers of America has a nice article describing the definition, pitfalls, warning signs etc. about subsidy presses:
Also known as vanity presses, royalty publishers or self-publishing there are some things of which you should be aware. In the world of blogs, Twitter, and other social media publishing has been transformed. Writers need a sharp eye for the changing face of publishing and to be aware of publishing options. The video on this page describes subsidy presses succintly and accurately.