What's in a name?
That which we call a rose
By any other name
would smell as sweet
-- William Shakespeare
What is a canonical URL and why should I care?
A canonical URL is the "official" name for a web page assigned by your website software. With CMS systems like WordPress or Joomla, a web page may be accessed by many terms.
For example, any of these URLs will display the same page:
https://www.pro-epic.com/index.php?option=com_content&view=article&id=148:how-to-be-consistent-with-your-content&catid=16&Itemid=5232
https://www.pro-epic.com/f1bf
https://www.pro-epic.com/blog/148-how-to-be-consistent-with-your-content.html
Google and other search engines will think that all the pages have duplicate content. That will decrease the ranking for all of the URLs.
The confusion can be eliminated by assigning a canonical URL with code like this:
<link href="https://www.pro-epic.com/blog/148-how-to-be-consistent-with-your-content.html" rel="canonical" />
Google will assign content ranking to only the page specified in the canonical tag, thereby conserving the rank of the "official" page.