scheme://host.domain[:port]/path/filename
where scheme is one of:
file
a file on your local system, or a file on an anonymous ftp server
http
a file on a World Wide Web server
gopher
a file on a Gopher server
WAIS
a file on a WAIS server
The scheme can also be news or telnet, but these are used much less often than the above. The port number can generally be omitted from the URL.