Four words, actually: International Standard Book Numbers
Every book sold through commercial channels (this does not mean you) needs a unique identifier for all the computer systems it’ll be entered into. While it’s a good idea to have an ISBN, you may not need to buy one. Many print-on-demand services will include one free; CreateSpace does. You’ll still be listed as the publisher, they just own the ISBN, meaning you can’t publish this exact book somewhere else—unless you get a new ISBN for it.
If you’ll only be selling the book privately (at your own events, from your own website, etc.) or if it’s for limited or private use, you do not need an ISBN.
If you’d like the control of taking your ISBN to another print-on-demand service or other printer or publisher, buy an ISBN from Bowker, the folks who have the monopoly on issuing ISBN numbers.
Consider your future plans when buying an ISBN. A single number costs $145 but a group of 10 costs $250. If you’re going to publish even two books, buy the set of ten and save $40. Use the extras to act as publisher for another author.