Hi,
When working with OpenID, what differences are there in having WWW in a domain name? I have noticed that some of the sites don't work when entered without the WWW part in the domain name. What is the reason for this?
Can this have an effect on OpenID delegation?
Short Answer
There is no difference, as long as the server the site is on allows both uses. For example, on my servers, I have a rule which says both using www and not using it are redirected to the domain without www (i.e., www.mysite.com will just automatically redirect to mysite.com).
Long(ish) answer
You might be wondering at the reason behind having www.
Basically, the domain name (i.e. the name of the server) is "my_site.com" or "my_site.org". In other words, the domain name is my_site, which gets translated to a specific server somewhere out there.
The thing is, domain names (and servers) have existed before the World Wide Web. One example of this is email: before there was such a thing as a "website", there was email (and message boards, chats, etc).
So when the World Wide Web came into existence, most servers were already in place and set up to do things like file transfers, email, etc. To differentiate between accessing the server for the purpose of visiting its website, and using the server for something else, the standard was to go to *www.*my_site.com.
Of course, nowadays, the Internet and the World Wide Web are pretty much the same thing (in the public's mind), so it's pretty standard for domains to work both with and without a "www" part.
Hope this helps clear up any confusion.
To follow up on Edan's answer.
It's more of a user than a system thing, the port number for htpp, gopher, ftp, smtp etc are all different so there is no need for different machine name.
When the www was new and a relatively minor use of the internet it was useful to put www in front of domains basically to advertise that you were doing this new fangled web stuff.