Domain names are organized hierarchically from right to left. The right-most portion of a domain name is called the top-level domain (TLD). The .com TLD is probably the most well known, but there are many others, such as .net, .org, .edu, and .mil. (Country codes, such as .us and .fr, are also TLDs.) When you register a domain name, you can choose which TLD to use (though some TLD's, such as .edu and .gov, are restricted to certain types of organizations).
The next domain name level after the TLD is called the second-level domain. This is the part of the domain name that you define. For example, xtraorbit is the second-level domain in xtraorbit.com. When you purchase a domain name, this is the portion that you “own”, as well as any subdomains beneath it. A domain name always contains a TLD and a second-level domain.
As you may have guessed, a subdomain is a third-level domain. The www subdomain is probably the most well known, but you can name subdomains anything you want. Subdomains are sometimes used to subdivide web site functionality. For example, you could have an English version of your web site at en.example.com, and a French version at fr.example.com.
If you want to register your own domain now, please follow this link.