We actually strip out the www. for your convenience, and http://www.sumofthyword.com/ will always redirect to http://sumofthyword.com/
www is an old protocol from back when everything was under separate server hardware. www (www.example.com) was the web server, mail (mail.example.com) was the mail server, ftp (ftp.example.com) was the FTP server, print (print.example.com) was the print server, etc.
These days, everything is run under the web server, making www. redundant and a waste of four characters.