It is a known fact that users tend to click on short URLs in the search results than their longer counterparts. However, if you have an old site, in all probabilities you would have multiple URLs pointing to the same product page. This includes some that longer exist because they included tracking information, category navigation, or other parameters.
Let's take an example:
Both the above URLs might lead to the same product page. So when Googlebot crawls a site, it's not unusual for it to find several URLs for the same page. And it's not always clear which of those should be displayed in the search results on Google.com.
To avoid this issue, Google now supports a new format that allows you to publicly specify your preferred version of a URL. 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.
This format uses the < link rel="canonical" > tag. For example, if you wanted to specify the shorter URL above, you could just have the following added to the < head > section at the top of your page:
< link rel="canonical" href="http://www.example.com/product?id=388" >
Now the Googlebot understands that the duplicates all refer to the canonical URL :
Additional URL properties, like PageRank and related signals, are transferred as well. And you can suggest a canonical URL on a completely different domain as now Google supports cross-domain rel="canonical" link element.
For more information on how to specify your canonical, check out our canonical URLs post on the Webmaster Central Blog.
The canonical URL you specify will indicate your preference for Google.com search results, but for Product Search, Product Listing Ads, and AdWords product extensions Google continue to use the URLs you specify in your feeds or through the API. That means that if you're submitting URLs with tracking parameters, they will continue to work.