Hello,
i'm often left wondering what's the significance of having "www" and not having it in the URL. i was further surprised to encounter certain URLs which had www2/3/...
i thought we have only one world-wide-web. and is the 'www' part just a symbolic inclusion, and something like www.abc.com and abc.com are one and the same thing?
www. is a subdomain just like any other subdomain, e.g. meta.stackoverflow.com
It emerged as a standard in the early days of the web and has stuck ever since, however it is completely optional and up to the web server to be configured to use the www. subdomain.
I think, abc.com, handles the DNS for domain abc.com. And, www.abc.com is the actual web server host inside that domain.
This web server host can have any hostname, just like we have mail.google.com.
In short: www is a convention in a hostname and does indeed stand for world wide web. Smart companies make sure that www.abc.com is the same as abc.com, but this is no requirement.
People sometimes use different hostnames (like www2.abc.com and www3.abc.com) to do load-balancing (e.g. distribute pictures over more than one server) or trick the browser to do more requests at the same time (this used to be limited to 2 concurrent connections per hostname)