When there are different choice for picking the same url, then picking the best url through a process which is called canonicalization . Suppose :- asd.com asd.com/index.html American School Directory all these are different for the server . So picking the best url for indexing is called canonicalization .
When Google discovers your webpages via multiple URLs, it is going to declare them as duplicate which is known as canonicalization. Hence, you need to tell Google about your preferred URL by adding rel=”canonical to your preferred version It does affect SEO since SE’s might rank the duplicate version of your website instead of the actual one. Here’s a complete guide to canonicalization http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139066 http://www.mattcutts.com/blog/seo-advice-url-canonicalization/
Canonicalization is the process by which search engines determine the best, or "canonical" version of a single page that can be retreived using more than one URL, and for different URLs that retrieve essentially identical content.“Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages.”
Canonicalization is the process of picking the best URL when there are several choices, and it usually refers to home pages. We need to redirect permanently to the original URL. For example, if we are targeting the www version of our website, we need to permanently redirecting the non-www version and .index.php to the www version. It helps to avoid duplicate pages.
As mentioned by Google, canonical URL is very similar to redirection since you are declaring a preferred or new location from an old URL.
