Google's Canonical tag

There are many different ways to type a URL...

o http://your-site.com
o http://your-site.com/
o http://your-site.com/index.html
o http://www.your-site.com
o http://www.your-site.com/
o http://www.your-site.com/index.html

Yes, they seem like small differences, but sometimes Search Engines treat each variation as a separate page, diluting your "link juice" to them. And then there are all the ways that sites might send you traffic, using slightly different dynamic URLs. And then, of course, you might have several pages that are very similar.

The canonical tag tells the engines...

This one. Use THIS URL. It consolidates all the links to the one URL. And it also tells engines that you are NOT using duplicate content tricks.