the search engine friendly URL that you want the search engines to treat as authoritative. In other words, a canonical URL is the URL that you want visitors to see.
Quite often canonical URLs were used to describe the homepage. The typical example used is that most people treat the following URLs as the same:
www.example.com
example.com
www.example.com/index.html
example.com/home.asp
The fact is that these are all different URLs. From a search engine perspective, this can cause a bit of an issue.