One of the important part of Search Engine Optimization (SEO) is the proper usage of keywords for meta tag. However, Google explained that Google doesn’t use the “keywords” in the meta tag for the web search engine ranking.

As per Google Webmaster Blog:

Recently we received some questions about how Google uses (or more accurately, doesn’t use) the "keywords" meta tag in ranking web search results. Suppose you have two website owners, Alice and Bob. Alice runs a company called AliceCo and Bob runs BobCo. One day while looking at Bob’s site, Alice notices that Bob has copied some of the words that she uses in her "keywords" meta tag. Even more interesting, Bob has added the words "AliceCo" to his "keywords" meta tag. Should Alice be concerned?

At least for Google’s web search results currently (September 2009), the answer is no. Google doesn’t use the "keywords" meta tag in our web search ranking. This video explains more, or see the questions below.

In summary, Google sometimes does use the description meta tag for the search result snippets, but Google still don’t use the description meta tag in the search engine ranking.