The preferred domain is the one that you would liked used to index your site’s pages (sometimes this is referred to as the canonical domain). Links may point to your site using both the www and non-www versions of the URL (for instance, http://www.example.com and http://example.com). The preferred domain is the version that you want used for your site in the search results.
To specify your preferred domain:
On the Webmaster Tools Home page, click the site you want.
Under Site configuration, click Settings.
In the Preferred domain section, select the option you want.
Note: Once you’ve set your preferred domain, you may want to use a 301 redirect to redirect traffic from your non-preferred domain, so that other search engines and visitors know which version you prefer.
