The International Standard Book Number (ISBN) is a unique numeric commercial book identifier.
An ISBN is assigned to each edition and variation (except reprintings) of a book. For example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN.
Some vendors, like CreateSpace, will acquire a "free" ISBN number for your project, But there are limitations to that method. For instance, a CS ISBN "owns" that ISBN, so if you move to a different self-publisher, you will need a new one.
Another way to obtain an ISBN number for your publication is to go to BowkerLink.com. When you purchase from Bowker, there are no limitations.
Bowker is the official source for ISBNs in the United States.
An ISBN uniquely identifies your book, and facilitates the sale of your book to bookstores (physical and digital) and libraries. Using ISBNs allows you to better manage your book's metadata, and ensure maximum discoverability of your book. Your book is listed in Bowker Books in Print®, which is used by all the major search engines and most bookstores and libraries.