Alrighty. So. If you want to publish a book, you need an International Standard Book Number, a little doohickey like this:
This little beauty will have to be on the cover of your book somewhere. It's required if you want to sell and distribute your books (or any written publication, really, although there are some exceptions). They're free in Canada, after you've registered with ISBN Canada, and you will use them when you put your book on Amazon or into bookstores, etc. But why?
Basically, ISBNs just helps the book industry keep track of all the books out there, so they can order and catalogue the books in bookstores, on Amazon, and elsewhere. It's numerical identification, and each format of your publication needs it's own unique identifier - so if you publish a novel, you need a different number for the hardcover, softcover, Kindle, Kobo, EPUB, etc., and also if you release any subsequent editions of the book (ex. revised edition, colour version, black-and-white).
This same number is valid for everywhere in the world where you plan to sell the book, so you don't need separate numbers for other countries (luckily, because even one novel could have a LOT of ISBNs for all the various formats).
When you register, you will be asked for information on the book; title, author, publisher, etc. If you're self-publishing, you can publish under an official company name (whether it's a registered and trademarked name or not), or, under your personal name. You can also change this information at any time, so don't panic if you haven't come up with your publishing company name yet.
Those are the basics, but of course, there could be variations from country to country. Check out your country's ISBN site and get started! For my fellow Canadians, go HERE!