I am frequently asked about whether to use the www or not in the URL. While many of us are predisposed to typing the www, most of the time including the www is immaterial as typing either www.mywebsite.com or mywebsite.com will usually take you to the same place. So which should you use and when?
The answer is either, but not both and here are specific cases to consider:
Declaring one or the other is important as search engines will see www.mywebsite.com and mywebsite.com as different websites and index each independently (Google lets you declare a preference in Webmaster Tools). This has the undesirable effect of dividing the rank for your content between the two sites. While this may seem odd, www is technically a sub-domain and legitimate sub-domains such as jobs.yourdomain.com, support.mydomain.com, and so on are common.
So why does it matter, wouldn’t having two versions of your site in the engine double you chances of being found?
Let’s say you have a product page called product.html and three clients place links on their websites to the page as www.mywebsite.com/product.html, while two others link to to the page as mywebsite.com/product.html. All other things being equal, if a competitor had five links to a single page www.competitorwebsite.com/product.html, the competitor’s page would outrank yours.
To avoid having a duplicate version of your website in the search engines, decide whether to use the www or not and create 301 re-directs (typing the undesired one will redirect to the desired version) to enforce that decision. Then add both to Google’s webmasters tools and declare the preference in site settings.