The major search engines (Google, Yahoo and MSN of course! 🙂 ) have announced that they will all support the use of the “canonical” tag as a way to understand duplicate content pages in a site. In essence, this means they are providing a way for webmasters to tell them which version of a page is the “master” and which are “alternative versions”. So, wherever you have the same content appearing in multiple pages on your site (say, a product on an e-commerce site that appears in several categories and hence has multiple addresses), you can tell the search engines that you know these are “duplicates” and which one it should treat as the “master”.
This is done with a simple line in the section <head> of your pages:
<link rel="canonical" href="https://www.website.com/category/product-page">
The canonical tag only works within a domain (including subdomains and folders) and should use absolute URLs rather than relative URLs (that means https://... addresses, not just ../path URLs).
Remember this will only work for pages that are very similar, so don’t think you can go applying the canonical tag to any page just to point some extra link juice somewhere else on your site!
There’s a slightly longer article about using these tags here at Search Engine Land.