Tags to describe various aspects about a Web page.
Meta tags may be used for a wide variety of metadata–data that describes other data–but many site promoters focus primarily on the description tag and the keywords tag. Some search engines use the description tag as the first choice for a search engine listing, using alternate methods if the tag is missing. Some search engines may also use the keywords tag to reinforce the main topics of the page. Not all search engine use meta tags, and the other ones may not pay much attention to the contents.
Meta tags that are hidden from the visible page, such as the keywords tag, have been subject to much abuse over the years. As a search rating factor gets abused, the natural reaction for the search engines is to de-emphasize the offending factor. This seems to be the case with meta tags, as search engines increasingly seek “reputation” factors less directly controlled by the web site owner.
see also: SEO Tools