Can anybody tell me the difference between these two?How and why we use 301 and 302 redirect?
Thnx...

301 is defined as "permanent" while the 302 is defined as "temporary"

301 and 302 both are redirection which means are given below:

301: Moved permanently,This and all future requests should be directed to the given URI.
302: Found, This is an example of industry practice contradicting the standard.

Very informative post. Thanks for sharing.

http://en.wikipedia.org/wiki/List_of_HTTP_status_codes

check this ....

301 Redirect

A 301 redirect tells the search engine that the page has moved permanently to the new URL.

302 Redirect

A 302 redirect tells the search engine that the move is only temporary, and you may decide to show content at the original location in the future without a redirect.

A 302 redirect says that you've moved a page or domain but only temporarily and the original page will come back.
A 301 redirect, on the other hand, tells Bing and Google that the page has been moved permanently and all SEO link equity should be transferred to the new page or domain.

301 tell the search engine that the page is move permanently to the new location. But the 302 tell the search engine that the page is moved temporarily to the new location.

301 is when you are moving permanently to a different web page, while 302 is only a temporary move. Most SEO experts only use 302 when testing or making adjustments to the current web page. It should be used carefully because mistakes can be very costly in terms of SEO. 301 is when you are telling the search engine that the move to a new URL is for good.

301 also mean dofollow
302 mean nofollow
People tend to go for 301

301 is defined as permanent redirection while the 302 is defined as temporary redirection of any webpage or website.

A 301, or permanently moved, is a redirect that alerts the crawlers that your content has moved permanently (that is, it's not coming back).

A 302, or "Found"/temporarily moved, is a redirect that alerts the crawlers that your content has moved temporarily (that is, it may come back later, like when you have it back in stock).

When it comes to SEO, a 301 passes the link information and updates the information in the search engine index so that the index will have the new address.  If you're moving a whole site over to a new domain (or whatever), then you should use this. You're still going to have a bit of time where things are kind of haywire in the index while Google (or whoever) reindexes everything, but it's better than you dropping out entirely.

A 302, well, it's not really a thing for SEO.  Sure, you can use it, but the crawlers will occasionally come back to check on things, because it's not permanent.  Whatever links, etc. you have coming to this page will continue to be all about this page and not the new page.  Again, this is for temporary stuff, which can happen, but usually not in the name of SEO.

Good luck.