There is a very good explanation of "canonical" at http://tinyurl.com/38ycpw8. by Jody Nimetz
The first part of it I have inserted here..... trust it helps:
Canonical URL: 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. Hence the idea of canonicalization. Canonicalization is the process of picking the best URL (to present to the search engines) when there are multiple choices available. Typically a search engine, such as Google will attempt to pick the best URL that they feel is the authority for that page. However, sometimes they may in fact select the wrong one. Now let’s suggest that you have product pages that depending on how the user navigated to the pager returns a different URL… same page but different URL, now we have a duplicate content issue. Not to mention the nightmare for interlinking and external link inventories.
The easiest way to avoid this is to let the Search engines and the users know which is your “preferred URL” a.k.a canonical URL.............................
