What the diferrent of www1 and www?
Thanks.

www1 would be somebodies way how to separate different servers with different sub domains

They have multiple web servers so they called them www1, www2, etc because you can't set up multiple web servers on the same domain name and name them all www because then there would be no way to tell which server to access.

@bappaditya
Are you saying "www1" is to www1.example.com as "mail" is to mail.google.com? Does this mean that "www" a subdomain but many modern web servers are able to assume a subdomain of "www" when none is given? (because "www.mail.google.com" doesn't give a response)

Thanks for clarification.

There is nothing significant about "www1".

It is just what some large sites have chose as their subdomain following the format of the - historically - default subdomain "www".

With subdomains you could pritty much have anything you want. www. is not required for nearly all web browsers and you can do quide a few subdomains (If your hosting provider allows it)
E.G.
google.com
gives the same result as
www.google.com
but there may be a backup server running on
www1.google.com
incase the main server goes down
its sort of like server 1, server 2 etc.
You could have a subdomain like
www.bob.homer.boris.yourdomain.com
and send it to a different server to
yourdomain.com

Hope at least some of that made sense