There is code in a web page that is not viewed by the web browsers but can be seen by the search engine spiders. This is the META code and more specifically the META Description and META Keyword tags. There are other META Tags that may be used but for the purposes of marketing and ranking a web site these are the two that come up the most.
META Description Tag is simply a summary of the the web page in 25 words or less. This is a great place to empasize the topic of the web page and use the main keyword phrases that the web page is targeting. There is much debate on how important the META Description Tag is but what it can help or hurt is in the search engines results pages (SERPs). Sometimes the search engine will use a part of or could use the whole META Description Tag for a web page that is ranking in the SERPs.
A good META Description Tag can influence a user to click on a listing. It can also make a user skip a listing if not well written. Is it important? Yes. Although we have seen sites rank with out, the META Description Tag is of moderate importance and unless your site has been indexed for years all factors are needed to make each page of your site rank on its own. Every META Description Tag must be unique to the page and summarize the page content.
META Keyword Tag offers only slight importance and mainly to Yahoo and a few other smaller search engines and directories. The days of spamming the META Keyword Tag are long gone. We use it to only emphasize our three or four main keyword phrases. It will not make or break the ranking of any web page. We have seen some sites use the META Keyword Tag to place common misspellings of the pages main keyword phrases.
Whether to use it or not is subjective but we will always include the META Keyword Tag in optimizing a web page. You never know if that one factor will make your page rank higher that your competitor if all other things are equal and they decided this tag was not important.