A 301 Redirect is an instruction to both Google, and to anybody who visits the site, that the content they are looking for has permanently moved to a new location, and it will redirect any visitors to the old URL to the new location.
This is in contrast to a 302 redirect, which is an instruction that the desired page has temporarily moved to a different location, and implicitly includes the fact that at some point in the near future, it will move back.
When To Use Redirects
The most common way to use 301 (permanent) redirects is when you have removed specific content or products from your website, and wish to send users who attempt to access those pages or products to a different page or product on the site. This can be useful when two sites merge, or when you move to a different site for example.
Another important use for them is during the conversion of a site from HTTP to HTTPS, when it’s necessary to redirect all traffic from the insecure version of the site, to the secure version, or if you have multiple domain names, but only one site, in which case all superfluous domains are redirected to the main site.
302 (temporary) redirects on the other hand, can be used for A/B testing, working on a live site, or getting feedback from a client without impacting rankings etc.
How Long Should You Keep A Redirect In Place?
In theory, a 301 redirect is a permanent redirect, and as such, could be kept in place permanently. However, when asked the question recently, Google’s John Mueller replied that although they should really be permanent, the best thing to do is to monitor those redirected URLs, and once there has been no traffic for a significant amount of time, it will be fine to remove them.
He recommended keeping redirects in place for a minimum of a year (and maybe longer), but once you are confident that no traffic is coming in to those old URLs, the redirects can be removed to help simplify server maintenance.
302 redirects however are designed to be temporary, and should only be in place for as long as necessary.
SEO And Redirects
According to Google’s Matt Cutts, a 301 redirect will not have any negative impact on your search engine rankings. Google will treat the new page as though it was the old page to all intents and purposes, although components of the new page which differ from the old one may impact that page’s ranking on their own.
Likewise, a 302, being temporary, shouldn’t have any impact either.
Just don’t use a 302 redirect when you mean to use a 301, and vice-versa.