If you need to change the URL of a page as it is shown in search engine results, we recommended that you use a server-side 301 redirect. This is the best way to ensure that users and search engines are directed to the correct page. The 301 status code means that a page has permanently moved to a new location.
301 redirects are particularly useful in the following circumstances:
- You've moved your site to a new domain, and you want to make the transition as seamless as possible.
- People access your site through several different URLs. If, for example, your home page can be reached in multiple ways - for instance, http://example.com/home, http://home.example.com, or http://www.example.com - it's a good idea to pick one of those URLs as your preferred (canonical) destination, and use 301 redirects to send traffic from the other URLs to your preferred URL.
- You're merging two websites and want to make sure that links to outdated URLs are redirected to the correct pages.
source :- http://support.google.com/webmasters/bin/answer.py?hl=en&answer=93633
