If you carefully read the page the shailender cited, you will find that it is precisely what you've inquired about.
Originally Posted by steveschmidt85
Did you perhaps not understand that rel="canonical" is not a complete tag in itself, but is rather a portion of a <link> tag?
Or, were you asking re. the effect of placing
on the page whose URL is http://www.xyz.com/blog/tag/abc? If so, the answer is that there is nothing to be gained by doing so, as the purpose of rel="canonical" is to ask that Google index a page under under a URL that is different from its actual one.
<link rel="canonical" href="http://www.xyz.com/blog/tag/abc" />
A commonplace use is to specify whether the page is to be indexed with or without the "www" service prefix.