For webpages which may duplicate some or all of their content with other pages (for legitimate reasons), the canonical URL is the URL of the page which search engines should link to.
For example, a particular product in an online shop may be reached through two different URLs, e.g. /product/18274 and /special-offers/18274. All pages should have the following HTML in the header:
<link rel="canonical" href="http://www.example.com/product/18274" />