Canonical URL and Canonical Tag
Canonical URL:- A canonical URL is specified URL to search engines that this URL is my preferred version URL. It means now you can specify all search engines (Google, Yahoo, MSN) that this is my preferred URL. If your website is showing similar content accessible from different URLs, for e.g.
* http://www.example.com/cotton?Shirt=male
* http://www.example.com/cotton?Shirt=male&sort_id
* http://www.example.com/cotton?Shirt=male&sort_id=good
than search engine will identify duplicate content on last 2 URLs. In this situation, It is necessary to specify search engines that this particular URL (http://www.example.com/cotton) is my preferred URL with the use of “Canonical Tag”.
Canonical URL Tag:- <link rel=”canonical” href=”http://example.com/page.html”/>
Now all major search engines (Google, Yahoo, MSN) has announced that they will support this “Canonical Tag” to specify your preferred URL to search engines. Now you can simply add this Canonical Tag under the <head> section on your duplicate URLs. By doing this search engines will understand that these duplicate URLs refer to this canonical URL and search engines will not consider these URLs having same content or duplicate content.
See Video for more information:-
Search Engine News-n-Updates 