There are two types of top-level domains: two-letter country domains, such as .uk (United Kingdom), and three-letter domains, such as .com, .org, and .net. National domain names follow the ISO 3166 standard two-letter codes for each country. The International Standards Organization adds new two-letter codes to the ISO 3166 list when the United Nations publishes an updated bulletin of country and region codes. You can learn more about this on the web site of the International Standards Organization.

Once a two-letter code has been assigned, the Internet Assigned Numbers Authority then identifies the responsible authority within that country that should be permitted to register subdomains within that country's domain. Some national domains, such as .tv (Tuvalu, a small island in the Pacific), have become available for commercial registration.

"Generic" domains, such as .com, .org, .edu and .net, are created and overseen by the ICANN (Internet Corporation for Assigned Names and Numbers). The original generic domains were .com, .edu, .gov, .int, .mil, .net, and .org. Additional top-level domains added in recent years are .biz, .info, .name, .pro, .aero, .coop, and .museum.

Any entity can register a domain in .com, .net, .biz, .info, and .org, although .org is typically used by nonprofit organizations and .net is typically used by Internet Service Providers. The .edu domain is reserved for accredited four-year academic institutions, and registration is handled solely by educause. .aero is reserved for the international aviation community, .coop is reserved for cooperative businesses, .museum is reserved for mseums, .name is reserved for individuals, and .pro is reserved for "licensed professionals," such as lawyers, doctors and accountants.

For more information about each of the generic domains and an extensive list of registrars available, see the ICANN accredited registrars page.