Description and Keywords Meta Tags should be used to give information about your site, but Keywords are not used by all search engines.
Description Tag
The Description Tag allows you to give a short description of your web page. If it is not included the search engines will take a snippit from the beginning of your page.
For a while
Google was not using the description meta tag.
The snippets on their search results were taken from the page content. At the moment they are using it again and most of the time it will be used for the snippet but its possible that sometimes the snippit will be taken from the content.
The following shows the snippit that Google took from the content. It would have been better if I had include a description in the Description Tag with more precise keywords:
You need to keep the description under 150 characters.
Search Engine Character Count for Description ( number includes spaces)
Google shows 156 Characters
Yahoo shows 161 Characters.
Bing shows 150 Characters
Ask normally does not use the description tag
Keywords Meta Tag
The Meta Tag for Keywords is not as important as it once was. Google no longer uses it but it is still used by other search engine and every little bit counts so you should include it.
Bing says:
The <meta> tag’s keyword attribute is not the page rank panacea it once was back in the prehistoric days of Internet search. It was abused far too much and lost most of its cachet. But there’s no need to ignore the tag. Take advantage of all legitimate opportunities to score keyword credit, even when the payoff is relatively low. Fill in this tag’s text with relevant keywords and phrases that describe that page’s content.Bing Community Blog