SEO and Canonicalization
Where SEO is concerned, the term canonicalization refers to normalization or standardization of the URLs for web pages that can be accessed through multiple URLs. For example, if a single page on your site can be accessed through three different URLs, then the popularity of the page is divided among the different URLs, and the page’s popularity is a fraction of what it could be. Canonicalization lets you tell the web crawler which URL is preferred for accessing 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