Many webpages which are dynamically generated such as those served from a CMS or e-commerce platform often have multiple URLs which load the same content. This can sometimes confuse search engines which often leads to a lower SERP (Search Engine Results Page) position and an incorrect URL to the page on your site.
Google, Microsoft and Yahoo have all recently adopted a new feature in their search engine spiders, canonical URLs. By adding an additional meta tag to any webpage you can indicate to the spiders your preferred URL for that page. A blog post from Google explains this feature well:
We now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version."
Matt Cutts, Google engineer
OmCore now fully supports canonical URLs. You can set your preferred URL in the site tree for any page under the metadata tab.