A canonical link element is a code level HTML element that helps webmasters and website owners prevent duplicate content issues by specifying the "canonical", or "preferred" version of a web page as part of the search engine optimization.
While the canonical link element has its benefits, Matt Cutts, who is the head of Google's webspam team, has claimed that the search engine prefers the use of 301 redirects over the canonical tag. Cutts claims the preference for redirects is because Google's spiders can choose to ignore a canonical link element if they feel it is more beneficial to do so.
The canonical link is used for SEO that will describe the ‘proper’ link URL that should be used in the search results like Bing and Yahoo and others.
You are selling a t-shirt that that shirt is named “Killer Whale Tee” and is located in Women’s, Sale, and Tshirt categories. In theory, there would be 3 URLs for each of those.