There is a tendency to refer to “POD Publishers” with disdain, but POD is just a printing technology. I use Lightning Source for printing, own my own ISBN numbers and retain all of my rights. I do my own design and layout. “Vanity Press” is the term most often associated with companies who offer book production packages, take a share of royalties or rights and bundle your work into their “publisher’s catalog”—and I think the more reputable vanity presses can be a good fit for many writers.
Lightning Source is a printer. iUniverse is a vanity press. Both use POD technology. I suggest a distinction between “POD Printers” and “Vanity Presses” with the term “publishing” reserved for those who own their own ISBNs, rights and royalties. If you publish through Xlibris or iUniverse, technically, you’re not self-publishing, but whether that distinction is important varies according to individual circumstances and points of view.
Irrespective of intellectual property considerations and who facilitates production, without POD, we’d all be sitting on stacks of books, handling fulfillment ourselves, and praying for the day when we get our closet space back.