Google gets angry when it finds a link to a page that no longer exists, and so do the users who click on the link that reaches a dead end. To fix that, we (marketers) find these old links and create bounces for them to the new pages.
Types of Redirections
There are three main kinds of redirects online; 301, 302 and meta refresh.
SEOMOZ, an online SEO tool, explains the three in the following way:
301 moved permanently
A 301 Redirect is a permanent redirect which passes between 90-99% of link juice (ranking power) to the redirected page. 301 refers to the HTTP status codes for this type of redirect. In most instances, the 301 redirect is the best method for implementing redirects on a website.
302 Found (HTTP 1.1) / Moved Temporarily (HTTP 1.0)
A 302 Redirect is a temporary redirect and passes 0% of link juice (ranking power) and in most cases should not be used. The Internet runs on a protocol called HyperText Transfer Protocol (HTTP) which dictates how URLs work. It has two major versions, 1.0 and 1.1. In the first version 302 referred to the status code 'Moved Temporarily'. This was changed in version 1.1 to mean 'Found'.
307 Moved Temporarily (HTTP 1.1 Only)
A 307 Redirect is the HTTP 1.1 successor of the 302 redirect. While the major crawlers will treat it like a 302 in some cases, it is best to use a 301 for almost all cases. The exception to this is when content is really moved only temporarily (such as during maintenance) AND the server has already been identified by the search engines as 1.1 compatible. Since determining if the search engines have identified this essentially impossible, it is best to use a 302 redirect.
Meta refreshes are a type of redirect that is executed on the page level rather than the server level (They are usually slower and not a recommended SEO technique). They are most commonly associated with a 5 second count down with text "If you are not redirected in 5 seconds, click here". Meta refreshes do pass some link juice but are not recommended as an SEO tactic due to usability and the loss of link juice passed.
SEO best practice
In order to maintain the SEO value, the 301 redirect is preferable as it will notify both browsers and search engine bots that the page has moved permanently. Search engines interpret this to mean that not only has the page changed location, but that the content, or an updated version of it, can be found at the new URL. The engines will carry any link weighting from the original page to the new URL.
Unfortunately, the other types of redirections (302 and meta refresh) will lose much of the original SEO value as they generally will not pass the rankings and search engine value like a 301 redirect will. Similarly, attempting to move entire domains can have a disastrous effect on SEO value, regardless of the type of redirect. For some great tips on how this can be avoided, visit Danny Dover's SEO Guide: How to Properly Move Domains.
Regex (regular expression)
Regex (regular expression) is a little piece of code to redirect a large amount of pages, rather than just a few. In technical terms, it is a string that describes or matches a set of strings, according to certain syntax rules, that is used to search and manipulate bodies of text based on certain patterns.
Regular expressions are a valuable skill to learn for both programmers and systems administrators. For a comprehensive tutorial on using regular expressions, visit Regular Expressions Info tutorial page.
Tools and resources
SEOMOZ Redirection; a comprehensive guide to webpage redirections.
How to create 301 redirects; examples of how to use 301 redirects in various programming languages.