A 301 redirect is a way of forwarding content that has been moved permanently from one webpage to another automatically. There are many redirect numbers so to speak, and the “301″ is referred to as “moved permanently”.  In laymen’s terms, creating a 301 redirect will prevent you from losing traffic that is still linking to content on an old URL on your site. The 301 redirect will forward or redirect that traffic to the content on the updated URL.  For example, if you have a web page that talks about SEO best practices, and you decide to move that content to a new URL under a new domain, you could create a 301 redirect so that anyone who still has access to the old content whether from a bookmark or cookie, they will be automatically redirected to the new content on the new URL.

301 redirects are important for SEO in several different ways:

1. Removed pages – Most people implement 301 redirects because they have removed a page from their site, but that page is still being indexed by the search engines; meaning people still can get access to that page. And instead of allowing your users to be served a 404 error page, you can simply implement a 301 redirect and point that removed page to another page on your site.   This will help prevent pissing your users off when they find removed content on your site but also to direct users to a relevant page in regards to their search.   Redirects will also help pass on any pagerank or link juice from your removed page to your new page, so that can help keep your rankings if you have any on that page.
2. Domain name changes – It happens, you start your blog on petexpert.blogspot.com and you later decide that you want your own URL at petexpert.com.  What do you do with your content that exists on your old URLs? You still have traffic going to those old URLs from referral sites, bookmarks, etc.  But you have this new content on your new URL already.  If you have the same content petexpert.blogspot.com AND petexpert.com the search engines will see that as duplicate content.  To a certain extent, you can be penalized for having duplicate content. To avoid this, create your 301 redirect from your old site, ‘www.petexpert.blogspot.com’ and point it to your new site ‘www.petexpert.com’. This way anyone linking to your old URL will now be automatically taken to your new URL with the same content. Once all traffic has subsided from your old URL, you can remove those pages and prevent them from being indexed. And in turn this will eliminate your duplicate content issues.
3. Multiple homepage versions – When businesses start up new websites, sometimes the hosting and domain companies will add in the www.petexpert.com/home ^[1] URL extension.  You may even see www.petexpert.com/index ^[2] or www.petexpert.com/default.asp ^[3].  These different URL’s are technically different web pages, and the search engines again can penalize you for having duplicate content.  I recommend adding a 301 redirect for any homepage extension URL to your main URL at ‘www.petexpert.com’ with no /home or /default extension.  This will tell the search engines to only go to www.petexpert.com ^[4] and no other page. And the more traffic you can get to your main URL, the higher your organic traffic can be.
4. WWW versus no WWW – Another issue identical to the one mentioned above is the www versus no www URL.  You may have your main URL be www.petexpert.com ^[4] but your webpage will also come up as just ‘petexpert.com.  I also recommend redirecting the no www to the www version.  This again tells the search engines to point to only one version of your homepage, not multiple versions which could count as duplicate content.

Now for the technical part, how do you go about implementing a 301 redirect you may ask?  BruceClay.com has a great post on how to properly implement a 301 redirect ^[5] that I recommend reading.

Again, the point of the redirect is to prevent having any issues with duplicate content as well as making sure you are telling the search engines exactly where to go to index the right page of your site. Also to help users find the right content they are looking for on your site and to pass on any existing page rank from  your removed page to your new page.