Redirects and SEO
Redirects are one of the things that people misunderstand when it comes to search engine optimization (SEO). The biggest problem is that they're often implemented incorrectly. Next in line is that most people don't know when to use them.
The first thing you want to do is to Google redirects for your particular web server software be it IIS, Apache, or something else. In addition you'll also need to look into how to pro grammatically implement redirects in the programming language of your site e.g. PHP, ASP, etc. What this article covers is when to use a redirect.
If you change the name of a web page, you need to set up a redirect from the old page to the new page. And it is important to use a 301 redirect as described in the article above. The 301 redirect will tell search engines that the old page has been REPLACED by the new page and they should stop going to the old page. Of course, it can take a while for search engines to adjust their databases so you need to keep the redirect around as long as possible. As a point of reference, over the years I have set up 250 redirects just for this site.
If you remove a page from your website you have the option of setting up a redirect so that traffic to that old page goes somewhere else. Sending people to your sitemap or home page are options that you should consider. If you do go down this path then set up the appropriate 301 redirects.
Things get trickier if you're changing domains. You might be inclined to, based on what you've read above, just set up a 301 redirect from the old domain to the new domain. There are two important things to consider though.
First, you want to redirect every page on the old domain to a unique and equivalent page on the new domain. Do not redirect all old URLs to the home page of the new domain. If you do, you may ultimately hurt your site when it comes to traffic from search results at Google, Yahoo, MSN, and other search engines. Plus it's a bad user experience since visitors to the old site will wonder why they're looking at the home page of another site.
Second, you actually don't want to do a 301 redirect. Initially, you should do a 302 redirect from the old site to the new site. Then monitor the new site to make sure the pages are included in the search engines. This can take a few months. Once you do see the new pages appearing in the search engines (particularly Google), switch the 302 redirects to 301. Doing this can help reduce the effects of the Google sandbox.