It is commonly known that you can link to your site with www or non www ( http://www.yoursite.com or http://yoursite.com ) as on most of server your site will open with both. So there is no difference from user's point of view as they will see the same site with both link structure. But the search engines will treat your website differently if web pages link to http://www.yoursite.com/ or http://yoursite.com/ as they can also be pointed to different destination. Search engine will treat one URL as real and inbound links on the other URL may not be counted. The subject is also known as Canonical URL Issue in search engine industry.