Along with the content, meta-tags have a lot of meaning that describes what the content is about. The most significant meta-tags are the keywords, description and robots tags.
The keywords meta-tag contains the main keywords that describe the document’s content. They have to precisely reflect the page content and not mislead the search engines. As webmasters, it’s a futile attempt to manipulate the search engines with the use of meta-tags. And, with the search engines constantly evolving, it’s not known for sure whether this tag has any impact at all anymore. We recommend not to refuse the usage of the Keywords meta-tag, though, as long as you use it reasonably. Use only those keywords that are really relevant to the page.
Example of keywords tag use:
Description meta-tag is a brief content (summary) of the document. Some search engines use text included into description in snippets on search engine results page.
Use example:
Webmasters can also use robots meta-tags that may forbid search engines to index the page (noindex meaning) or count the links (nofollow).
[...] Smart Viper Development Blog Just another WordPress weblog « Use of meta-tags [...]