The terms self-publishing and POD are often used interchangeably: but they are quite different.
Self-publishing is the process by which a writer publishes their own work, usually in book form.
POD is short for “print on demand” (and not, as some vanity presses insist, “publish on demand”), a printing process which utilises digital technology to print books one copy at a time.
It is possible (and usual) for the self-publisher to use POD technology to print their books, but it is not a requirement.