An author who decides to self-publish basically becomes the publisher. You provide the funds required to publish the book, as well as the camera-ready artwork. You are responsible for marketing and distributing the book, filling orders, and running advertising campaigns. In the past, the author had to decide on the number of copies to print, sometimes resulting in unsold books. The Print on Demand (POD) technology now used by some self-publishing companies means that authors can have fewer copies printed—only as many as they need.
With self-publishing, depending on the company used, you can have a finished book - hardcover or paperback or both – within a couple of weeks after the manuscript is completed. And, with an e-book, this can be reduced to days. Of course, you are responsible for managing these services, and making sure the books have the desired look and feel, and are up to your desired quality level.
The self-publishing author pays for everything - design, layout, printing, advertising, distribution - to get the book into retailers and ultimately into people's hands. The major payoff for all of that payout is control. You own the content, the title, the price, and all decisions about promotion and distribution. You also receive the revenues from the book sales, after the retailer takes their cut.