Canonical
Why Specify a Canonical Page
A canonical page is the preferred version of pages that have very simular content. One page might have products that are listed by price and another page the products are listed alphabetically. The search engines might only index one of the pages. You can select the page that you want to be the canonical.You need to create a <link> element which you will place in the <head> of all of the none canonical pages and then reference the canonical page.
<link rel="canonical" href="http://www.example.com/womenshoes.php"/>
womenshoes.php would be the canonical page.
Using this method is recognized by all of the major search engines but it is recognized as a suggestion rather than a directive.
Which URL?
When the search engines index sites they see urls with the prefix www and without www as two completely different sites, this confusion can change your Page Ranking because back links can be going to both examples of the domain name or additional urls for the same site. For example you may have:
http://example.com
http://www.example.com
http://example.com/default.php
http://example.com/default.php?news=example
In order to clarify that all of these urls are for the same domain you need to create a 301 which is a permanent redirect. If anyone requests http://example.com they would be then be redirected to http://www.example.com.
Matt Cutts in a Google video states that this helps them identify the url which you would prefer to be canonical.
Please Note:.
In the aforementioned video Matt Cutts also states that it is preferable if the site has absolute urls for its internal links. But having absolute internal links will slow your site's load time and also prevents the owner from changing domains without changing all of the links. The owner may not want to change domains but most often when a site is in creation it is tested on the web designer's domain, this would not be possible if the internal links were absolute. I have checked a number of sites that have high page rankings and find that most have internal links that are relative. Even companies where the creation and testing of the site would most likely be done in house. Two such sites are Apple and W3C.