Self-publishing does not mean you are actually using a printing press to print a book. Self-publishing is the act of writing, editing, marketing, and printing your own work. As a self-publishing author, you pay to publish and promote your book. You are considered an independent publisher unless you have signed away the rights to the words you have written. Being an independent publisher gives you a solid basis that can be used to begin an independent publishing business of your own.
Self-publishing has been going on for many years. There are a number of authors who have submitted their works to publishers time and time again, before getting fed up and publishing their works themselves. There are many great literary pieces that have been self-published. Mark Twain self-published Huckleberry Finn and after being rejected by several publishers, Richard Paul Evans decided to self-publish the well known holiday book, The Christmas Box. Richard Nelson Bolles originally self-published his book, What Color is Your Parachute, but later Ten Speed Press took over the publishing. This is not unusual in the world of self-publishing. You may originally publish your book, but after it is out there it becomes of interest to a publishing house.
Self-publishing is a business and should be approached as such. It can be a very lucrative business if handled properly or you can lose a lot of money if handled improperly.