@brad_jonesMar 10.2016 — #The search engines don't like duplicate content and thus if you have an article that is in multiple places or on multiple sites, then the search engines won't know what copy is the primary article and which are just copies. Thus, there is a chance that all the pages (and sites) will get penalized for duplicating content. The Canonical tag lets you tell the search engine which copy of an article is the master or primary copy of the article. All copies of an article should have a canonical tag that points to the original.
There are few site things more frustrating than to have a copy of an article rank higher than an original copy that you might have created. ? Canonical tags help avoid this.
Brad has kindly answered your query, but please use Google and/or Bing to research a question like this before raising a thread. It took less than a second to get the answer by searching "html canonical tag".
@RH-CalvinMar 11.2016 — #The rel=canonical tag passes the same amount of link juice (ranking power) as a 301 redirect, and often takes much less development time to implement. The tag is part of the HTML head of a web page.
@brknnyMar 28.2016 — #canonicalization refers to individual pages that can be loaded from numerous URLs. This is an issue since when numerous pages have the same substance however distinctive URLs, interfaces that are proposed to go to the same page get split up among various URLs.