A top level domain, also known as a TLD, is a domain at the top of the Domain Name System hierarchy.
Under the Domain Name System, an Internet address has four elements; a server prefix, a domain name, a domain suffix (or extension), and a country code (the only optional element). A TLD or top level domain, therefore, would consistently follow the naming convention of something such as "http://www.susanward.com".
For years, there were only a few top level domains, such as .com (for commerce), .gov (for government), and .org (for organization). (You can see how the Domain Name System (DNS) simplifed domain names; if you saw .gov as part of the website address, you knew you were dealing with a government site.)
However, because of the explosive growth of the Internet, there's been a corresponding growing need for new top level domains. ICANN (The Internet Corporation for Assigned Names and Numbers) has added quite a few new top level domains to the DNS.
Currently, domain names under the following general Top Level Domains are available: .aero, .biz, .cat, .com, .coop, .info, .jobs, .mobi, .museum, .name, .net, .org, .pro and .travel.
Besides TLDs, there are also country code top level domains (ccTLDs) in the DNS. These are specific to particular countries or geographic regions, such as .ca for Canada and .uk for the United Kingdom.
