means that the URL requested has moved permanently and has been assigned a new URL. Any future requests should use one of the returned URLs. The search engines prefer the use of a 301 redirect over temporary 302 redirects or a meta refresh. If you have duplicate content on your site, you will want to place a 301 redirect from the duplicate pages to the main destination page.  Times when you want to use a 301 redirect include if you are moving pages around in sub-folders, changing actual files names or are changing domains.