With the canonical tag, you can specify your preferred version of any page URL.The canonical tag for each page will be different, based upon upon how you wish to identify the standard version of a page URL. In accordance to the specification set by the search engines, the canonical tag can use an absolute or a relative URL path in the tag.
Canonical tag is used to let the search engines know the original source in case you have duplicate content pages. e.g. root page ends with / or /index.php have the same contents but different urls.
Google can reach your web page through different URLs, this may create a confusion of duplicate content for search engines. To avoid this, we choose a preferred path for the web page (or canonical path) for the web page. With canonical URLs you tell your preferred version of URL to the search engines. Read this help page about canonical tags for more information, http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394
Great information, Thanks to all, but from above all posts I find the conclusion is that, Canonical Tags use just for to avoid duplicate content issues.
Canonical tag is basically used to tell prefer URL to search engines. Example <link rel="canonical" href="http://www.domain.com"/>
this tag eliminates duplicate content issues from the same domain or also from other domain also please advice,,
categorize means if the content is duplicate and also comming in copyscape. then through canonical it will be benefit