I see some sites with urls such as
www6.example.com, but I don't know what
Does anyone know what this means and if it's different from a URL that just has
The www part of a url is just a subdomain of the domain name. www is common but there's nothing special about it other than everyone knows about it. The people who run example.com could just have easily used wwwsix or wwweight, or secure.example.com, mail.exmaple.com, etc. They could use different sub-domains for different countries, like us.example.com, or fr.example.com.
"www" is just the name of the host or server for the website. You can have any name you like for your webserver, such as "mywebhost.example.com" but "www" is now such a well-known and understood convention for a publicly available worldwide web site that it would be very unusual to do so other than for very well known sites or internal-use hosts (such as an Outlook Web Access site - you might choose to use owa.example.com for your users to visit, separate from your company website).
So, since this is just a host name essentially it can be anything. Sites with multiple front end servers would usually hide these behind a single round-robin name such as www, but it is possible that you will be redirected to a specific host for things like downloads or media streaming. In this case www6 or www8 are probably just two servers in a large farm of hosts providing material to users on the worldwide web.
It just means someone has no imagination and someone never come up to read the proper guide on domain usage: http://no-www.org/
In fact, it's more of first issue. Instead of calling it srvr8.example.org or serv8.example.org, etc, they use what's first on their minds: www. It is deprecated. Domains working only on www always make me a bit mad.
In some cases it even comes to me sending a mail to site admin to review no-www.org. :)