Every WebSite is referenced to a Domain Name, also know as a URL, or Universal Resource Locator. This Domain Name is the address people type into their browsers to find your site. But there are many new places where you can find them a bit cheaper. The reason you want to get your domain name before you even start your site, is because it's the first to go. You see, if someone gets the domain name you want first, they will have the rights to it for as long as they renew it.
Domains are available with a variety of extensions.
The other three most common extensions are .net, .org, and .edu. While .mil, .gov, and .edu are regulated, the restrictions on the other domain names are nothing more than good intentions. Anyone can buy a .net and use it for whatever they want. There are countless domains left out there. If you have an unusual name, there's a good chance that it's still available.