A canonical page is a URL that is treated by search engines as an original, preferred, non-duplicate version of a page.
When a page's content can be accessed via multiple URLs, or when a group of pages contain content that is very similar to one another, duplicate content problems can occur.
To avoid these issues, webmasters should tell search engines which URL in the group should be treated as the preferred version (the canonical page), and which URLs are merely duplicates or near-duplicates (non-canonical pages). This can be accomplished using the rel=canonical tag. Non-canonical pages will not be indexed by search engines, but canonical pages may be.
A page is considered to be canonical if there is no rel=canonical tag on the page, or if the rel=canonical tag points to itself.
In the examples in the table below, the first URL is not canonical since the rel=canonical tag points to a different URL. The second is a canonical page since the rel=canonical target points to the same URL. The last is a canonical page since there is no rel=canonical tag, search engines assume that this page contains original, non-duplicate content.
URL | Rel=Canonical Target | Canonical Page?
http://www.example.com/about-us - N/A - Yes