You probably already know about the description and keywords meta tags. But there are lots and lots of meta tags that you can add to your website. Some are great for your management of your pages and others provide information for external programs (including Web browsers, servers, and robots).
Meta Tags for Site Management
Site management meta tags are used mostly by the people working on the website. While they may be interesting for your customers, they are typically more important for you and anyone editing your pages.
Who wrote this Web page? You can include a list of authors if multiple people wrote the content and it typically refers to the content authors rather than the designers of the HTML or CSS.
<meta name="author" content="author name" />
Set the copyright date on the document. Note, you shouldn't use this instead of a copyright notice that is visible on the Web page, but it's a good place to store the copyright in the code as well.
<meta name="copyright" content="© 2008 Jennifer Kyrnin and About.com" />
This is a contact email address for the author of the page (generally). Be aware that if you put an email address in this tag, it can be read by spammers, so be sure to protect your email address.
<meta name="contact" content="email address" />
When was this document last edited?
<meta http-equiv="last-modified" content="YYYY-MM-DD@hh:mm:ss TMZ" />
Meta Tags for Communicating with the Web Browser or Server
These meta tags provide information to the Web server and any Web browsers that visit the page. In many cases, the browsers and servers can take action based on these meta tags.
Control how your pages are cached. The options you have are: public (default) - allows the page to be cached; private - the page may only be cached in private caches; no-cache - the page should never be cached; no-store - the page may be cached but not archived.
<meta http-equiv="cache-control" content="no-cache" />
Define the natural language(s) used on the Web page. Use the ISO 639-1 language codes. Separate multiple languages with commas.
<meta http-equiv="content-language" content="en,fr" />
This meta tag defines the character set that is used on this Web page. Unless you know that you're using a different charset, I recommend you set your Web pages to use UTF-8.
<meta http-equiv="content-type" content="text/html;charset=utf-8" />
If the content of your page has an expiration date, you can specify this in your meta data. This is most often used by servers and browsers that cache content. If the content is expired, they will load the page from the server rather than the cache. To force this, you should set the value to "0", otherwise use the format YYYY-MM-DD@hh:mm:ss TMZ.
<meta http-equiv="expires" content="0" />
The pragma meta tag is the other cache control tag you should use if you don't want your Web page cached. You should use both meta tags to prevent your Web page being cached.
<meta http-equiv="pragma" content="no-cache" />
Control Robots with Meta Tags
There are two meta tags that can help you control how Web robots access your Web page.
This tag tells the Web robots whether they are allowed to index and archive this Web page. You can include any or all of the following keywords (separated by commas) to control what the robots do: all (default) - the robots can do anything on the page; none - robots can do nothing; index - robots should include this page in the index; noindex - robots should not include this page in the index; follow - robots should follow the links on this page; nofollow - robots should not follow links on this page; noarchive - Google uses this to prevent the page from being archived.
<meta name="robots" content="noindex,nofollow" />
Google has their own robot - GoogleBot, and they would prefer that you use the googlebot meta tag to control the Googlebot. You can use the following keywords to control the Googlebot: noarchive - Google will not display cached content; nosnippet - Google will not display excerpts or cached content; noindex - Google will not index the page; nofollow - Google will not follow the links on the page.
<meta name="googlebot" content="nosnippet,nofollow" />