What is an ISBN?

An ISBN is a unique identifier attached to your books, which you purchase from Bowker. This unique identifier is most often seen on print books and is how your barcode is formed, though it also allows listing, purchase in shops and more. Almost every country (bar, at the time of writing this, Canada) requires you to purchase them, but you can purchase one, ten, 100, or 1000.
If you use an ISBN, you need one for your Ebook, and if you create them, Print, Audiobook.
See also ASIN.