The article How Web Servers Work discusses the concept of domain names. "Howstuffworks.com" is a domain name. The com portion of the name is called the top-level domain name. There is a very small set of generic top-level domain names, which you will probably recognize:
There are also a number of top-level domain names for individual countries, including countries like Canada (ca), Australia (au) and the United Kingdom (uk).
IANA, the Internet Assigned Numbers Association, provides world-wide management of the top-level domain names. See the next page for more information and related links.