URL

Uniform Resource Locator ( also refered to as a URI though not exactly the same)

  protocol://dns.address:port/path/to/requested/information

Protocol://

This is most commonly http://, https:// or ftp://. But can be smb://, afp://, rss://, etc.

Please note that file:/// is used in places and the extra / refers the root of the local file system

dns.address

Is either a fully qualifed domain name (FQDN) or can be a short domain name if your local name service resolver is configured to try certain sub domains.

:port

This is always optional and is always a numeric value, eg. :8000

/path/to/requested/information

This is optional but most of the time you will see some path information appended to your [URL]