The W3C does not define a set of legal metadata properties (META Tags). The meaning of a property and the set of legal values for that property are defined in a reference lexicon called a profile. For example, a profile designed to help search engines index documents might define properties such as author, copyright, keywords, etc.
<meta name="keywords" content="META Keywords Tag, Metadata Elements, Indexing, Search Engines, Meta Data Elements">
The META Keywords Tag is where you list keywords and keyword phrases that you've targeted for that specific page. There have been numerous discussions at various search engine marketing forums surrounding the use of the keywords tag and its effectiveness. The overall consensus is that the tag has little to no relevance with the major search engines today, but...
How do I improve the ranking of my web site in the search results?
Use a "keyword" meta-tag to list key words for the document. Use a distinct list of keywords that relate to the specific page on your site instead of using one broad set of keywords for every page.
Relevance of the META Keywords Tag is minimal. The keywords tag may be considered in the overall equation of page quality depending on the user agent.
If the keywords or keyword phrases do not appear on the page visibly, then they should not appear in the keywords tag. We recommend that they come from your title element, meta description, heading and first paragraph of content, in that order.
Historical Note: For those search engines that are META enabled, the META Keywords Tag used to be one of the most important areas after the TITLE Element and META Description Tag. The current ongoing claim is that because this tag has been abused over the years by marketers and consumers alike, the search engines have decreased the relevance of the keywords tag. In some circles, the keywords tag has been deemed irrelevant and/or deprecated. I'm still not 100% certain about that especially with Yahoo!'s 2009 Small Business Search Help documentation. Also, couldn't that abuse argument be applied to any page element and/or attribute?