SEO and Canonicalization
In SEO, canonicalization means picking a standard URL when a page is available through different links. For example, if people can get to the same page on your site by three URLs, each link splits the page’s popularity, so it won’t be as popular as it could be. Canonicalization lets you tell the web crawler which URL you prefer people to use for the page. For more information, see URL Component Aliases.
These same principles hold true if visitors can access your site by using different domains. For Example http://www.mywebstore.com and http://mywebstore.com both take a visitor to the home page of your site. SEO best practice states that you should choose only one and be consistent in its use. For more information, see Canonical Domains