Defnition:Canonical Tag

When referring to programming, canonical means conforming to well-established patterns or rules. The term is typically used to describe whether or not a programming interface follows the already established standard.

Google, yahoo  and MSN has come to a conclusion on Canonical Tag.Search engines will consider this as a Guidence to them, not as a directive, Unlike “No Follow Tag”

Where ever the duplicate or repeated content is there on your webpages, place this Canonical Tag, But not on the original page.

Eg: example.com/abc.com

Example.com/abc1.html

Exmaple.com/abc2.html

Exmaple.com/abc3.html

If abc2.html and abc3.html has same content which is there on abc1.html, place the canonical Tag  on this pages.Exclude on abc1.html. This guides search Engines to consider abc1.html as primary information. Abc2.html and abc3.html also supports to abc1.html.This will work in a similar way to 301 redirect, routs all the page rank to the original page.

How to Use canonical tag

Place the canonical tag on the duplicate pages, such as repeated product pages or Old content pages, this gives an indication to the search engines Not to consider these repeated pages as duplicated ones.

Specify the canonical version using a tag in the head section of the page as follows:

Syntax:

<link rel="canonical"href="http://www.example.com/product.php?item=swedish-fish"/>
For its potential usage, use follow the link mentioned below.

http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html