For webpages which may duplicate some or all of their content with other pages (for legitimate reasons), the canonical URL is the URL of the page which search engines should link to. This tag is used for canonical 301 redirects and other methods. See the [rel-canonical] tag for more information on rel="canonical".
For example, a particular product in an online shop may be reached through two different URLs, e.g.
One way of specifying which is canonical is by using rel-canonical tags. All pages should have the following HTML in the header:
<link rel="canonical" href="http://www.example.com/product/18274" />
Another way of specifying which is canonical is by implementing 301-redirects from the non-canonical URL to the canonical URL: