I've joined a few writer's groups on Facebook and I've been very surprised by what some writers don't know. One topic that came up is ISBNs. So we're going to talk about ISBNs.

What is an ISBN?
ISBN stands for International Standard Book Number. It is the number on the top of the picture up there. One agency per country is designated to issue ISBNs. ISBNs can be found on any type of media world-wide and are unique identifiers for works. They can identify different editions, as well as different titles.
How do I get an ISBN?
The agency approved to issue ISBNs in the US is Bowker. You can visit their website at myidentifiers.com. DO NOT BUY ISBNs FROM ANYONE OTHER THAN THEM.
Why?
Every ISBN is registered to a person or company. If you are buying your ISBN from a third party (publisher, individual, etc.) that ISBN is associated with your book, but it DOES NOT belong to you. It belongs to the third party you purchased it from.
Why is this bad?
Inherently, it isn't. If you're looking to save money, buying an ISBN through a publisher is often an easy thing to do. They are often able to buy in bulk, so they can make ISBNs cheaper to you. BUT, if anything were to ever happen to the publisher, say they go out of business, you will lose your ISBN. ISBNs are crucial to you as an author. You cannot sell your book without one.
What should you do?
That is completely up to you! If you want to buy through a third party, you can. Just be sure to do your research about who you're buying from. However, you should always try to obtain your own, if possible. This protects your work and your investment. You will never lose your ISBNs for your book, although you may pay more money. There is a form that must be filled out to register your ISBN. It can be a bit tricky, but don't panic! A quick Google search will often resolve your questions.
And of course, never be afraid to reach out to other self-published authors for help, including me! I'm so happy to answer questions and to make sure that you are successful and not getting scammed! I know better than most how it feels to be a little fish swimming in a big confusing pond.