"gov" is one of the top-level domain names that can be used when choosing a domain name. It generally describes the entity owning the domain name as a branch or an agency of the U.S. Federal government. (Other U.S. government levels are encouraged to use the geographic top-level domain name of "us".) Along with the second-level domain name (for example: "whitehouse" in whitehouse.gov), the top-level domain name is required in Web and e-mail addresses.
For more information, see gTLD (generic top-level domain name).