Domains and links to your site can occur with or without the www in the domain such as http://longest.com and http://www.longest.com. Search engines can view these as two different domains. It is speculated that this may effect your PageRank with Google and potentially rankings with other search engines.
The thought is that if you have 20 links to a page using the www and 20 without, they may be looked at as two different pages and therefore only 20 links are associated to the page as opposed to 40. If 40 links were associated you may obtain a higher search engine ranking.
Google calls this a domain preference and allows you to set your preference. Log into Google’s Webmaster tools. Then navigate to the Tools option on the left hand menu, then to the Set preferred domain option. You will see a screen like below and you can choose, it is defaulted to Don’t Set an Association but this is where you can choose with or without www.
It is that simple, cannot hurt, and hopefully over time will help.
Why It Matters for Search Engine Optimization
It has been said that the version of your domain with and without the “www” can be seen as two different sites. Someone may link to your site or an article/page on your site using the http://www.longest.com and someone else may just link to http://longest.com. In that case search engines may view them as two different sites and you will not benefit from both links. Companies such as the rankpay search engine optimization firm talk about setting the preferred domain because you would want your site to get the benefit of both links. That may no longer be the case however it does not hurt to set the preferred domain just to be safe.