A 301 redirect is a way to tell a search engine that a page has moved to a different URL. A 301 redirect is different from other redirect methods because a) it is permanent, and b) it allows for any “link juice” or search engine authority that has been built to that specific page to be transferred to the new page’s URL.
An example of a common use of a 301 redirect is when a website is being redesigned, and the URL’s on the newly redesign website are different from that of the old website (wither because the page names are different, or because the new website uses an old page extension from that of the old one, such as .html, .asp, .cfm, etc.).
If the old website’s pages had multiple links built to those pages, than a 301 redirect should be implemented in order to keep the amount of authority that the old web pages had so that the newly designed website can keep it’s amount of authority with Google and other major search engines. If 301 redirects are not implemented, then the redesigned website will likely lose it’s search engine rankings, and the number of visits being registered in Google Analytics and other traffic monitoring tools will likely be lower when it comes to organic search engine visits.
For more information on 301 redirects check out:
Commonly Made Mistakes When Redesigning a Legal Website (by George Murphy of The Search Ninjas)- Avvo Blog
301 Redirects– Google Webmaster Tools
Redirection- Best SEO Practices -SeoMoz
Check for 301 Redirect Tool– WebConfs
How to create a 301 redirect -WebConfs