[ NOTE: My position on this is now different, but I kept the post for historical purposes. I still do agree with parts of it as well. ]
It’s no longer necessary to use “www” when referring to websites. It’s a waste of storage, a waste of ink, and it takes 2-5 times as long to pronounce as saying the domain by itself. Enter no-www.
No-www is an initiative to make all websites accessible from both the http://www.example.com/ and http://example.com/ forms of their names. The reason behind it is to standardize domain names providing web content and to avoid typing unnecessary letters. — Wikipedia
The reason for using the “www” hostname prefix when entering websites is now a matter of history. It’s old. Deprecated. Outdated. Antiquated. Like websites that only work in Internet Explorer, sites that break when you use the domain alone should be firmly encouraged to join the 21st century. It’s wasteful to type, and it’s cumbersome to pronounce. Consider that it takes nine syllables to properly enunciate three characters.
Some argue that there is still a good reason to separate traffic destined to web servers from that destined to the domain itself. I think this is precisely the idea that’s become obsolete. The reason for that differentiation in the past was that the other protocols were as prolific (or even more prolific) than HTTP at the time. HTTP was just one of many options, so it made no sense to give any type of traffic a preference by pointing the root of a given domain to the system that hosted that content.
Things are different now. HTTP is utterly dominant. Sites interacting with the public on any serious scale does so via a web server, so making this the default is a matter of obvious practicality. This doesn’t mean we should abolish the use of hostnames. Hostnames are excellent tools for separating traffic and making meaningful associations with users. The argument here is simply for having the root point to the web content as a matter of convention.
As for people who include it when speaking, there’s no need to be rude when correcting them. Just kindly inform them that it’s faster if they just go to the domain itself, and that the “www” isn’t needed.
For more information, check out the no-www FAQ.