The Rel Canonical link element solves the duplicate content issues related to a website. It is an HTML attribute directing the search engines about the main or canonical version of the web page. It works well when there are several pages on a website having the same content. In this kind of situation, you can use the rel=canonical attribute to guide the search engines as to which page is your main page.
URL Canonicalization :
Suppose you are having an eCommerce store selling mugs online. You have provided an option for sorting the mugs via price, color or material. The unfortunate situation is that sorting would use the same group of mugs in the database again and again. Only the urls would change (&sort=price, &sort=color, &sort=material etc) but the content would more or less remain the same. In this situation, we apply the rel=canonical tag to prevent all these urls from correlating with your main product page and direct the search engines to use your main product page as the main canonical page.
<link rel= canonical href= http://www.canonicalexample.com/thisiscanocialurl.php /> In this example given above, the url is the main version of the several duplicate pages in your website.