OK, technical stuff time. If you want to be able to change where you send people when they click on a link, you might want to use a URL redirect. For example, you might be promoting product A on your web site (or in a press release, or wherever) but that product becomes no longer available, so you want to switch to promoting a similar product, product B.
By setting up a redirect URL at the start, which sends people to product A, you can later easily switch it to redirecting to product B.
There is an excellent article on the SEOMoz site about redirects. Here is an excerpt:
The first common example of this takes place with a simple scenario: a URL that needs to redirect to another address permanently.
There are multiple options for doing this, but in general, the 301 redirect is preferable for both users and search engines. Serving a 301 indicates to 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, as below.
Be aware that when moving a page from one URL to another, the search engines will take some time to discover the 301, recognize it, and credit the new page with the rankings and trust of its predecessor. This process can be lengthier if search engine spiders rarely visit the given web page, or if the new URL doesn’t properly resolve.
Other options for redirection, like 302s and meta refreshes, are poor substitutes, as they generally will not pass the rankings and search engine value like a 301 redirect will. The only time these redirects are good alternatives is if a webmaster purposefully doesn’t want to pass link juice from the old page to the new.
Transferring content becomes more complex when an entire site changes its domain or when content moves from one domain to another. Due to abuse by spammers and suspicion by the search engines, 301s between domains sometimes require more time to be properly spidered and counted.