Usually, writer submits an unsolicited manuscript (MS) to the publisher through an agent or "over the transom," (directly). The publisher reads the MS and decides to publish it. Sometimes the publisher "commissions" a writer to write a book. In either case, the publisher prints, promotes and distributes the book.
In self-publishing the writer contracts with a publishing house to print the book, and the author promotes and distributes it. Sometimes the publisher offers services like proof-reading, editing, etc. AuthorHouse is a good self-publisher. You can also contract with a commercial printer, but you don't get the additional services.
