Links may point to your site using both the www and non-www versions of your site's URL (for instance, http://www.example.com and http://example.com). Google sees both as two different web pages if your site is not using redirection from www to non www or vice-versa. This might hurt your link building efforts and might also get you penalized for duplicate content because Google sees both URL pointing to same content.
To be on the safer side, it is highly recommended that you set your preferred domain in Google Webmaster Tools. Setting the "preferred domain" enables you to tell Google if you'd like URLs from your site crawled and indexed using the www version of the domain (http://www.example.com) or the non-www version of the domain (http://example.com).
Once you set your preferred domain name, Google considers all links that point to your site to be pointing at the version you prefer. For instance, if you specify your preferred domain as http://www.example.com and Google finds a link to your site that is formatted as http://example.com, Google will follow that link as http://www.example.com instead. In addition, Google will take your preference into account when displaying the URLs in search results..
How Do I Set My Preferred Domain?==> Go to www.google.com/webmasters
==> Add and verify the www and non www versions of your site's URL. Tip: try using the same verification code that you used to verify your first domain. That way you’re just one click from verification
Once both have been verified, go to the Google Webmaster Tools homepage and click on the site you want.
Click the gear icon and click "Site Settings"
In the Preferred domain section, select the option you want and SAVE.
NOTE: It may take some time before you see this change fully reflected in Google index. All pages that currently display the non-preferred version of the URL in the index will remain in the index that way until Google refreshes their indexing of those pages.
Have you set your preferred domain name? Which one do you prefer? (www or non www). Kindly let me hear from you via comments.