URLs which begin "www2"

www2 most likely means that that you have been sent to their 2nd web server directly -- possibly they do not have load balancer properly configured .. or it is configured this way on purpose, so once redirected user stays there and not hitting balancer again.

Shopping.com does it all the time (at least did it in the past).

It also could mean that it is temporal redirect -- their main server is currently unable to handle the load (or not working completely) and you have been redirected to backup/secondary server for the time being. It is possible that if you visit this www2 URL next time (in few hours/days/weeks) you may be redirected back to their main web server (www).

To answer the last question - if you have a domain then you can set up any subdomains you like www2.domain.com, www3.domain.com etc.

WWW prefix
Many domain names used for the World Wide Web begin with www because of the long-standing practice of naming Internet hosts (servers) according to the services they provide. The hostname for a web server is often www, in the same way that it may be ftp for an FTP server, and news or nntp for a USENET news server. These host names appear as Domain Name System (DNS) subdomain names, as in www.example.com. The use of 'www' as a subdomain name is not required by any technical or policy standard; indeed, the first ever web server was called nxoc01.cern.ch, and many web sites exist without it. Many established websites still use 'www', or they invent other subdomain names such as 'www2', 'secure', etc. Many such web servers are set up such that both the domain root (e.g., example.com) and the www subdomain (e.g., www.example.com) refer to the same site; others require one form or the other, or they may map to different web sites.

From here: http://en.wikipedia.org/wiki/World_Wide_Web#WWW_prefix