The difference between a 301 and a 302 is that a 301 status code means that a page has permanently moved to a new location, while a 302 status code means that a page has temporarily moved to a new location.
Using 302 redirects is a dangerous practice. Search engines don't like this redirection type because it is a common strategy that spammers use to get more of their domains up in search engine results. Another reason to use 301 redirects instead is that then your URLs maintain their link popularity. If you set up 302 redirects, Google and other sites that determine popularity ratings assume that the new link is eventually going to be removed. After all, it's a temporary redirect. So the new page doesn't have any of the link popularity associated with the old page. It has to generate that popularity on its own.
If you're changing your site's domain name, you should never use a 302 redirect. This almost screams "spammer" and is a good way to get all your domains blocked from Google and other search engines. If you have several domains that all need to point to the same place you should use the 301 server redirect. This is common practice for sites to buy additional domains with spelling errors (www.gooogle.com) or for other countries (www.symantec.co.uk), and then redirect them to the primary Web site. As long as you use a 301 redirect, you won't be penalized in search engines.
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