Canonicalization

Canonicalization is the process of picking the best URL when there are several choices, and it usually refers to home pages:
• www.domain.com
• domain.com/
• www.domain.com/index.html
• domain.com/home.asp
• http://home.domain.com
• www.home.domain.com
Technically all of these URLs are different. A web server could return completely different content for all the URLs above. When a search engine “canonicalizes” a URL, it’s trying to pick the URL that seems like the best representative from that set.
Don’t make half of your links go to http://domain.com/ and the other half go to http://www.domain.com/. Instead, pick the URL you prefer and always use that format for your internal links.
Let’s say you want your default URL to be http://www.domain.com/. You can modify .htaccess file so if someone requests http://domain.com/, it does a 301 (permanent) redirect to http://www.example.com/. That helps search engines know which URL you prefer to be canonical. Adding a 301 redirect can be an especially good idea if your site changes often (e.g. dynamic content, a blog, etc.).