Some sites, especially those using CMS, generate multiple URLs for the same page. This can cause duplicate content and be penalizing in terms of SEO. A canonical URL is the one that we want to keep among these multiple generated links. To clean up and thus free up space in their indexes, the engines agreed on a tag allowing to specify for each page the canonical URL …

 duplicate-content- search engine

A tag to avoid duplicate content…
CMS can generate this kind of URL: www.monsite.com/ma-page.html.sid=asdf321165656565, the latter returns exactly the same as the following URL: www.monsite.com/ma-page.html. If for the visitor that does not change much, for the engines, the page can be indexed twice. It is therefore important to specify to the engine that it does not need to keep these two URLs in its index and that it can keep only one and unique, which is called Canonical URL, in our case : www.monsite.com/ma-page.html.

Training & Co'm

In practice, a tag must be inserted in the HEAD of the page in question to indicate this to the engine. It will take this form:

 

Around which the engines have agreed …
The three main engines have agreed on this new tag which should help them to have cleaner indexes.

If you want to know more, each of the engines communicated on this subject:

Google: Specify your canonical
Yahoo: Fighting duplication adding more arrows to your quiver
Microsoft: Partnering to help solve duplicate content issues

For people with a blog running with WordPress, a plugin has already emerged: Canonical URL for WordPress!