Uniform Resource Locator ( also refered to as a URI though not exactly the same)
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
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.
This is always optional and is always a numeric value, eg. :8000
This is optional but most of the time you will see some path information appended to your [URL]