http://www.websitedesign.co.uk

http://www.websitedesign.co.uk/index.php

http://websitedesign.co.uk

http://websitedesign.co.uk/index.php

and so on will all resolve to a single page, yet Google will see each as an individual resource. This can be a problem for a number of reasons. First is that Google will make an arbirtary decision about which one of these will be the ‘main’ page for it’s index which may not agree with your choice, and second, that the power of your incoming links can be dissipated if sites linking to use use different URL’s.

Google has addressed this problem by asking webmasters to add this tag to the head section their pages:

This link tells Google that http://www.websitedesign.co.uk should be considered the ‘main’ URL for all of the URL’s that resolve to the home page of our site. Now, with the canonical link in place, all the links that point to our home page will boost the link popularity for the single URL http://www.websitedesign.co.uk.

For full information about canonical, check ou this page on Google Webmaster Help.