1) <Description> metatag: Metatags are becoming less and less important but if there are metatags that still matter, these are the <description> and <keywords> ones. Use the <Description> metatag to write the description of your site. Besides the fact that metatags still rock on Bing and Yahoo!, the <Description> metatag has one more advantage – it sometimes pops in the description of your site in search results.
2) <Keywords> metatag: The <Keywords> metatag also matters, though as all metatags it gets almost no attention from Google and some attention from Bing and Yahoo! Keep the metatag reasonably long – 10 to 20 keywords at most. Don’t stuff the <Keywords> tag with keywords that you don’t have on the page, this is bad for your rankings.
3) <Language> metatag: If your site is language-specific, don’t leave this tag empty. Search engines have more sophisticated ways of determining the language of a page than relying on the <language>metatag but they still consider it.
4) <Refresh> metatag: The <Refresh> metatag is one way to redirect visitors from your site to another. Only do it if you have recently migrated your site to a new domain and you need to temporarily redirect visitors. When used for a long time, the <refresh> metatag is regarded as unethical practice and this can hurt your ratings. In any case, redirecting through 301 is much better.