The domain name tells you the type of organization sponsoring a page, which can help give you insight into the quality or reliability of the information. It is a three-letter code that is part of the U R L and preceded by a "dot." Here are the most common domains.

.com commercial entity
Companies advertise, sell products, and publish annual reports and other company information on the Web. Many online newspapers or journals also have .com names. In terms of choosing the best academic sources, commercial sites may not be the best places to get your information.
.edu educational institution
May be more reliable than a commercial site. Be sure what you're looking at has been put up by a professor or the university, though. Watch out for student papers!
.gov government
Federal and state government agencies use the Web to publish legislation, census information, weather data, tax forms and many other documents.
.org non-profit organization
Nonprofit organizations use the Web to promote their causes. These pages are good sources to use when comparing different sides of an issue. Since there is no commercial motivation, may be more reliable than a .com site.

There are also .net for internet service providers and .mil for U.S. military. In addition, more top level domain names were added in 2001.