Some people type www.google.com into their Web browsers, and some people type google.com instead. What do you type? How do your Webs react to what others type?
Well, I was just hanging out at www.yes-www.org (doesn't work due to a missing www record in his DNS zone, ironically enough: here's a local copy until he gets his DNS fixed, or you can just manually tell your Web browser to connect to his IP address (74.217.45.98) manually and request www.yes-www.org, since his Web server is doing just fine as I type this) on a referral from no-www.org (which makes the laughable claim that the hostname "www" has been deprecated).
I read through some of the comments, and realized that even so-called "technical people" are often rather badly misinformed. This page is an attempt to clear up some of the confusion.
First of all, it's important to understand the difference between a host and a domain. A host refers to a (physical or virtual) server, while a domain refers to a logical grouping of hosts, domains, and/or other resource records. In a typical Web-serving domain configuration, you can expect to find something like this in the zonefile (the list of resource records for a zone (== domain)) for domain.com:
That record refers to a (physical or virtual) server called www, which makes it pretty obvious that the server is there to serve your little contribution to the WWW.www.domain.com. A 12.34.56.78
You might also have mail servers at your domain. A POP3 server might have a hostname pop3, an IMAP server might have a hostname imap, and a webmail server might have a hostname webmail:
It might be more convenient to have a single mail server (named mail) with POP3, IMAP, and webmail services all running locally:pop3.domain.com. A 12.34.56.80 imap.domain.com. A 12.34.56.81 webmail.domain.com. A 12.34.56.82
You might also have a mail server setup to receive incoming mail. In addition to the host (A) record, you'll also want a mail exchanger (MX) record, in such a case:mail.domain.com. A 12.34.56.83
You might have a public FTP server within your domain, as well:mx1.domain.com. A 12.34.56.84 domain.com. MX mx1.domain.com.
...and perhaps an IRC server:ftp.domain.com. A 12.34.56.85
Obviously, each of the above servers might have a Web server running on it (for a Web-based management interface, or maybe a Web-chat or webmail service). Put another way, going to http://ftp.domain.com/ may yield an HTTP interface to the public FTP server's contents (perhaps for people who don't have an FTP client handy, or people who are behind a firewall and don't have an FTP client supporting PASV, or maybe ftp.domain.com itself doesn't support PASV, or just for people who don't like FTP, but still want to browse the files on your public FTP server). http://mx1.domain.com/ might be a control panel for your incoming mail server. http://irc.domain.com/ may be a Web-chat interface to irc://irc.domain.com/. ftp://www.domain.com/ may also be useful to your Webmaster, who most likely wants to manage the pages on the www.domain.com server, rather than the files stored on your public FTP server. ftp://irc.domain.com/ may have archives of recent activity in your IRC channels. In fact, ports 25 (SMTP), 110 (POP3), and 143 (IMAP) on irc.domain.com may cooperate to provide an email-based interface to your chat server. Clearly, once you're prepared to learn how the Internet works, the possibilities are endless.irc.domain.com. A 12.34.56.86
Now, if you'd like to provide a service to people who try connecting to a hypothetical host named as your domain itself, you're certainly free to do so:
You could then install a Web server at 12.34.56.87 that might respond at port 80 with a list of hyperlinks to interesting stuff at domain.com. (You could also install services at other interesting ports (25, 110, 143, etc.) on that server, redirecting (or proxying, for protocols that don't have redirection capabilities) to the correct server.) Now, if the only (or "main") interesting thing to do at domain.com is http://www.domain.com/, you might want to save your users some time, and redirect them automatically to www.domain.com, rather than showing them a menu with only one choice (basically, the rough equivalent of Google's I'm Feeling Lucky feature). If there's little other than Web service at your domain, or if virtually everything interesting at your domain happens on 12.34.56.78 anyway, then you might want to consider doing away with the complexity of a whole zoneful of hosts, and just have a simple A record for your entire domain:domain.com. A 12.34.56.87
In such a case, www.domain.com wouldn't really have any meaning, although you might want to provide aliases (CNAMEs, or canonical names) to honor age-old conventions:domain.com. A 12.34.56.78
It's worth noting that even in such a case, it may prove worthwhile to keep hyperlinks pointing to http://www.domain.com/, since you might grow to the point where you might want to have http://bbs.domain.com/ being a Web-based interface to your BBS (a.k.a. forum) service, http://irc.domain.com/ being a Web-based interface to your IRC (chatroom) service, and/or irc://support.domain.com/ being an IRC interface to your customer support system.www.domain.com. CNAME domain.com. ftp.domain.com. CNAME domain.com. ;assuming you're providing FTP service irc.domain.com. CNAME domain.com. ;assuming you're providing IRC service
Now, I keep hearing people saying that you should just do http://domain.com/ in order to "dumb down" the WWW for non-technical people. I've always thought the obvious answer to that was that one should never artificially dumb something down, because trying to oversimplify one thing inevitably results in overcomplication of another (for example, the "very simple" (and very expensive, in a server environment; SPOF and IOS upgrades come to mind) NAT setup that a home user pointed out in one of the comments - just to avoid giving individual servers individual names), but reading through people's comments, it looks like this basic principle of Information Theory seems to be one of the most closely guarded secrets of our universe. I guess the best answer is that people were never meant to be dumbed down (e.g. the discovery of the Pythagorean Theorem without the aid of our modern numbering system, or the invention of Computer Science centuries before the first computer), and anybody who'd like to be artificially dumbed down to the point of not even having a "www" in front of my domain probably wouldn't find any of my content terribly interesting in the first place, so it's "no great loss," in both directions. (You can't possibly expect a monkey to enjoy a Web intended for humans, so it's silly to alienate the humans just to lure the monkeys.)