Meta tags are a great way for webmasters to provide search engines with information about their sites. Meta tags can be used to provide information to all sorts of clients, and each system processes only the meta tags they understand and ignores the rest.
Meta tags are added to the section of your HTML page. Google can read both HTML and XHTML-style meta tags, regardless of the code used on the page. With the exception of verify, case is generally not important in meta tags. This is not an exclusive list of available meta tags, and you should feel free to use unlisted meta tags if they are important to your site. Just remember that Google will ignore meta tags it doesn't know.
Read more at https://support.google.com/webmasters/answer/79812