.ca : Canada
.jp : Japan
.uk : United Kingdom
.us : United States
and many more.
To send electronic mail to someone over the Internet, all you need to know is their "username", or "handle". This is followed by an at-sign, the node name, and any domain names. Thus
joe@locoweed.chi.il.us
donotes a private citizen in Chicago, Illinois, in the U.S. Joe's computer is called "locoweed".
As another (real) example,
president@whitehouse.gov