The 'www.' part is just to point out that's it's a URI or whatever (at least that's what I think, without research of the actual reason) maybe to avoid confusion with an actual '.com' file (even though there is a large array of domains today).
Many people type the 'www.' part so it's within the DNS zone file to make sure it still points to your site regardless if it's there or not.
WHM = Web Host Manager
Looks like Webdisk could be a sub-domain of some kind that allows modification to your accounts storage without FTP. I assume that it would be used in cPanel's file manager.