www

It didn't start out that way. The names separated by periods signify (potentially) different machines on the Internet. Originally, www was a shorthand for the host running the web server software. In large companies this is a separate computer from the one running mail or other servers. The toher parts are the company / entity name (e.g., mycompany) and the top-level domain (com or org). There's a database of mappings from these human readable names (e.g., mail.mycompany.biz) to IP address which are four octets (numbers from 0 - 255) again separated by periods (e.g., 192.168.0.1).

The protocol part of the URL (or URI) is http when the transport protocol is going to be the HyperText Transfer Protocol. Others are ftp (File Transfer Protocol) or SMTP (Simple Mail Transfer Protocol). The syntax for an HTTP URI/URL is:

protocol://host.domain.top_level_domain[:port]/path_to_file (Don't click on this link, my example syntax fooled the posting software into treating this as a link.)

The port part of the URI is optional, and defaults if not given to 80.

That being said, most web clients (the software you use on your computer to download and render HTML web pages is very forgiving of malformed or abbreviated URLs.

Ceci n'est pas un seing.