There are two types of redirects that you need to be familiar with as a webmaster: Permanent redirects and temporary redirects. 301 redirects are the permanent kind and the most often used. 302 redirects are the temporary redirects. Here’s when you should use them:
301 Redirect – The permanent redirect is best used when you are redesigning your site and you want all of your URLs on one domain name to point to all of the corresponding URLs on another domain name. For instance, you could redirect all traffic from http://www.mydomain.com to http://mydomain.com and if you use the 301 redirect then you are sending all of your traffic to the new domain name on a permanent basis.
Another benefit to this type of domain name is it transfers all of the historical data (PageRank, link popularity, etc.) to your new domain name. So you don’t lose any site authority with this type of redirect. It is often used when a webmaster or company decides to change its domain name for branding purposes. In other words, if you wanted to change your domain name from http://www.mydomain.com to http://www.keywordspecificbrandname.com then you could use a 301 redirect.
302 Redirect - The 302 redirect is a temporary redirect and does not transfer PageRank, link popularity, or any historical site search engine optimization authority to your new domain. This type of redirect is best used if you are running a holiday promotion and you want to send all of your traffic to a holiday promotion landing page. So you’d use the 302 redirect to send traffic from http://mydomain.com to http://mydomain.com/holidaypromotion. After the holidays, you’ll remove the redirect and all traffic will go to your index page as normal. You will still retain your historical data as long as you don’t maintain the redirect for too long (ie a couple of years).