A stupid question

Nothing, some people just type in the name, some type in www.
It's best to have only one way though.
You can use things such as HTACCESS to move everyone from the www to just http, or the other way around

it's different names but usually both of them configured together in apache domain aliases.

Starting the domain name name of your web site with w_w_w or www2 or www3 is a common convention and nothing more. There is nothing in the HTTP specification that says a web site must start with www or any other prefix. It is simply a convention that began in the early days of the web and was used to distinguish a company's web server from its FTP server, gopher server, mail server, et cetera. But no such distinction is necessary, because web servers respond on TCP port 80, which is distinct from port 25 (outgoing mail), port 110 (POP mail), port 21 (FTP control), et cetera. A single domain name can host all of these services, and a single physical computer may actually respond to many different domain names.

As a matter of common practice, most webmasters make sure their sites respond to both w_w_w_mycompanyname_com (I'm not allowed to post urls) and mycompanyname_com, because the former is the first guess many people will make, and the latter is both more convenient to type and easier to fit into an advertising logo. It is rare for a well-run web site to reject either name.

Some web sites use www2., www3., www4., etc. as names for additional web servers that handle some of the traffic for the site. But it's strictly up to the webmaster and the person who handles DNS for the domain. These could just as easily be catnip.example.com, dirtnap.example.com or whatever else.

To make domain.com goto w_w_w.domain_com, put this in your .htaccess file, located in public_html. Download and open in a text editor.

Thanks guys. Thanks a lot.