If you plan on selling your book in a brick-and-mortar bookstore, or an online bookstore (like Amazon or Barnes & Noble) you will need an ISBN number and barcode. A barcode is a graphic of vertical lines that can be scanned for sales or inventory. The ISBN number is used to create the barcode, but the number and the barcode are two separate things.
The International Standard Book Number (ISBN) is a 13-digit number that uniquely identifies your book as a specific title from a specific publisher (you, as self-publisher). If you are selling your book outside of stores through your own website or through back-of-the-room sales at speaking engagements, an ISBN is not necessary. However, if you plan to sell your book with online booksellers, in bookstores, or put it in libraries, then it is advisable to purchase a number.
Visit MyIdentifiers.com to purchase your ISBN. The charge for a single number is $125.00 and a block of ten numbers is $295.00. Each edition of your book (hardcover, ebook, softcover) needs a separate ISBN number. You can purchase the ISBN from the website, and receive it by email right away.