Google does not use the "keywords" meta tag in its web search ranking. Google's Matt Cutts explains this in a Webmaster Central video. This is not breaking news, by any means, but there are a lot of people out there that still put a lot of stock into this.
Search engines pay attention to some tags, and none to others. Remember when the 'keywords' meta tag mattered? Ah, the good old days.
Meta Tags were once a major player in SEO. With the advancement of search engine algorithms, meta tags become less and less significant. The description meta tag is still used for your search engine snippets, but the keywords meta tag has been disregarded by all the major search engines.