Domain names (often simply called domains) are easy-to-remember names associated with particular IP addresses. For instance, the domain name example.com might translate to 198.102.434.8. This makes it much easier to remember a website's Uniform Resource Locator (URL).
Examples of domain names are google.com and wikipedia.org. Owners of these domain names can create different subdomains such as www.google.com, docs.google.com, www.wikipedia.org, and en.wikipedia.org.
You can register a new domain name with Google Apps or use an existing domain name you already own by verifying domain ownership.