To prevent duplicated versions of a page from being indexed we want to use canonicals on our site.
Many webmasters use tracking parameters and or ads as well on the sites they manage. Almost all of the time it will have a ? in the URL and after the ? is the tracking code.
The canonica for this page is –
<link rel=’canonical’ href=’http://www.mattstorms.com/2013/08/rel-canonical-website/‘ />
If I wanted to have tracking on this page I could have the URL to be
If you look in the source code on line 51 the canonical renders as
Now to the reason of why you want to have the canonical tag on your site. The purpose is so that you do not duplicate your content with tracking code. This keeps potential internal spam from showing up in the search results.
Hope that helps understand what a canonical is and what you use it for, if you have any questions please go find me on Google+.