This is a little technical, but I like to let you know when Google does something important.
Actually, this is Google, Yahoo and Msn – the three largest search engines, responsible for close to 90% of all searches – agreeing on a new “tag”. A metatag is a behind-the-scenes piece of information ABOUT the website or a page in it, such as its title, description, keywords, etc.
Tags are usually invisible to someone viewing the site. They are communications or instructions to search engines or web browsers.
One issue the search engines have is that the same page may be reachable in a number of different ways, actually resulting in different page names. Search engines hate this because they don’t want to clutter up their indexes. But they have (until now) had no way of knowing which is the preferred page name.
The canonical tag provides a way to tell the search engines what name a page should be indexed under.
To learn more about the canonical tag.