What is a 3xx Redirect?
3xx redirects are URLs that redirect to another URL. The "3xx" is referring to a range of HTTP Status Codes that all instruct browsers and search engines to follow a redirect to a new URL.
There are a number of reasons redirects might be used:
Change of Domain - If you change domains, you'll want to redirect each page from the old domain to the new one.
New URL - If you need to change the URL of a page for any reason within the same domain, you should do a redirect from the old location to the new one.
Multiple URLs - You may use multiple URLs to access the same page content. However, it's not a good idea to have multiple copies of the page, since they may be treated as duplicate content. Instead, redirect all of the URLs to one central URL.
Removed / Merged URLs - Much of the time, when you remove a page from your site you can set up a soft 404 page for the site, so search engines know this page is gone and they can remove it from their index. However, sometimes you may want to remove a page and use an existing page to substitute it. For this, we can remove the old page content and redirect its URL to the substituting page's URL.
Why It's Important
If this page contains valuable content on it but is redirected by mistake, it may be removed from search engines' index and no longer rank. If this page is redirecting to the wrong page, it could confuse website visitors and hurt search engine rankings.
What is a 301 Redirect?
This type of redirect is used to signal that the redirect is permanent - the old URL will not be used again. This is generally the type of redirect you want to use on your site. That's because 301 permanent redirects typically pass all the link juice from the first URL to the target URL. Therefore, when a page moves, it's possible to keep the same ranking position with the new URL, thus preserving your SEO efforts. It's also a signal to search engines that the original URL is no longer used, so they will generally remove it from their index.
What is a 302 Redirect?
A 302 redirect is a method of redirecting from one URL to another. Search engines generally interpret this as a signal that the redirect is a temporary phenomenon and could change back at any time. Because of this, search engines may not remove the original URL from their index and may not pass any link juice to the target URL since they expect this situation to be temporary.
This means that if a high-ranking URL moves to a new domain or a new location somewhere else on your site, all of the hard work you've put into getting the first URL ranking well will not be passed on to the new URL location, which can be disastrous for your site's rankings.
This means that if a high-ranking URL moves to a new domain or a new location somewhere else on your site, all of the hard work you've put into getting the first URL ranking well will not be passed on to the new URL location. This can be disastrous for your site's rankings, and therefore is not advised in most situations.
Other 3xx Status Codes
The vast majority of 3xx redirects are either 301 or 302, but there are other less commonly-used redirects as well.
A 303 status code means "See Other", and is generally used for web applications. It's not actually a redirect exactly, but instead a notice telling the application that the correct response can be found at a different URL.
304 is "Not Modified", which again is not exactly a redirect but a message from the web server that the page has not been changed since last time accessed or cached.
While 303 and 304s are far less common, 305-308 are even more rare or not used at all.
See the Wikipedia entry for HTTP Status Codes for more detailed information on each type of redirect.
What Should I Do With This Data?
Check to make sure each URL redirects to the correct target URL you intended
Ensure the target URL of each redirect does not return a 4xx or 5xx error
Verify there are only 1-3 redirects in a chain. Search engines may stop following redirects after 4 in a row.
Make sure 301 redirects are used instead of 302s for all URLs that you want the ranking and link value to pass to the target URL