Thanks for your question! This is a fairly common question as people often believe that the WWW is a preferable way to state the URL. Depending on who you ask these days, the answer can actually go either way.
However, by DEFAULT, the server is set to already understand the domain name as eitheer www.domainname.com or just domain name.com (the www is set to be a CNAME which is an alias that automatically points to the domain name).
However, if you wish to FORCE one or the other to appear when the domain is displayed in the browser, then you will need to make a change in the .htaccess file of your site. Check out this article on forcing it using the .htaccess file: Force WWW in >htaccess.
I hope this helps to clarify the question. If you require further assistance or have any further questions, then please let us know.
Thanks Arn! That is exactly what I needed to know!