Google added a new “Parameter handling” feature to Webmaster tools to assist you with managing duplicate content and allows you to view which parameters Google believes should be ignored or not ignored at crawl time, and to overwrite our suggestions if necessary. With the "Parameter Handling" setting, you can now provide suggestions to our crawler to ignore the parameters category, trackingid, and sessionid. If we take your suggestion into account, the net result will be a more efficient crawl of your site, and fewer duplicate URLs.
- rel="canonical" has to be put on each page, whereas parameter handling is set at the host level
- rel="canonical" is respected by many search engines, whereas parameter handling suggestions are only provided to Google