Canonical URLs relate to the concept of selecting the best URL for the web pages that the visitors want to see. Also, known as canonical tags, these URLs help in content syndication when multiple versions of a same page become available over the Internet. Thus, it is used to resolve issues related to content duplication.

For example, most people would consider these the same urls:

www.rendc.org

rendc.org/

www.rendc.org/index.html

rendc.org/home.asp

But technically all of these urls are different.