Fancy words aside, a canonical URL is Google’s way of identifying a “preferred” URL for your posts to avoid duplicate content. Duplicate content is generally defined as “separate web pages with substantially the same content, which may attract a penalty from search engines.”
WordPress is often criticized for having duplicate content since new posts appear on many pages including category pages, archive pages, feeds, and trackbacks. While this helps visitors find the content they are looking for, it confuses search engines, forcing them to “choose” which URL to serve in search results.
And so… Google (Yahoo and Microsoft too) recently came out with a new link tag to help with the duplicate content issue which can be added to the <head> section of the duplicate content URLs.
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
But honestly, who can be bothered to go into the <head> for every post. Luckily, there are 2 WordPress plugins that are here to help:
Yoast adds rel=”canonical” links to your blogs <head> section
SEO No duplicate – This simple plugin helps you easily tell the search engine bots the preferred version of a page by specifying the canonical properly within your head tag.
For more information, read the official announcement from Google.