Meta Tags are important to your site, but just which ones are used, and which ones aren't.

<meta name="robots" content="…, …">
<meta name="googlebot" content="…, …">
These meta tags control how search engines crawl and index the page. The "robots" meta tag specifies rules that apply to all search engines, the "googlebot" meta tag specifies rules that apply only to Google. Google understands the following values (when specifying multiple values, separate them with a comma):

• noindex: prevents the page from being indexed (see "Block or remove pages using meta tags")
• nofollow: don't follow links from this page when looking for new pages to crawl (also see "Block or remove pages using meta tags")
• nosnippet: don't show a snippet of this page when displaying it in the search results (see "Prevent or remove snippets")
• noodp: don't use text from ODP (The Open Directory Project a.k.a. dmoz.org) to generate a title or snippet for this page (see "How do I change my site's title and description?")
• noarchive: don't display a "Cached" link for this page in the search results (see "Prevent or remove cached pages")
• unavailable_after:[date]: remove this page from the search results after the specified date and time (see "Robots Exclusion Protocol: now with even more flexibility")

The default rule is "index, follow" -- this is used if you omit this tag entirely or if you specify content="all." Additional information about the "robots" meta tag can be found in "Using the robots meta tag." As a side-note, you can now also specify this information in the header of your pages using the "X-Robots-Tag" HTTP header directive. This is particularly useful if you wish to fine-tune crawling and indexing of non-HTML files like PDFs, images or other kinds of documents.

<meta name="google" value="notranslate">
When we recognize that the contents of a page are not in the language that the user is likely to want to read, we often provide a link in the search results to an automatic translation of your page. In general, this gives you the chance to provide your unique and compelling content to a much larger group of users. However, there may be situations where this is not desired. By using this meta tag, you can signal that you do not wish for Google to provide a link to a translation for this page. This meta tag generally does not influence the ranking of the page for any particular language. More information can be found in the "Google Translate FAQ".

<meta name="verify-v1" content="…">
This Google webmaster tools-specific meta tag is used on the top-level page of your site to verify ownership of a site in webmaster tools (alternatively you may upload an HTML file to do this). The content value you put into this tag is provided to you in your webmaster tools account. Please note that while the contents of this meta tag (including upper and lower case) must match exactly what is provided to you, it does not matter if you change the tag from XHTML to HTML or if the format of the tag matches the format of your page. For details, see "How do I verify my site by adding a meta tag to my site's home page?"

<meta http-equiv="Content-Type" content="…; charset=…">
This meta tag defines the content-type and character set of the page. When using this meta tag, make sure that you surround the value of the content attribute with quotes; otherwise the charset attribute may be interpreted incorrectly. If you decide to use this meta tag, it goes without saying that you should make sure that your content is actually in the specified character set. "Google Webauthoring Statistics" has interesting numbers on the use of this meta tag.

<meta http-equiv="refresh" content="…;url=…">
This meta tag sends the user to a new URL after a certain amount of time, sometimes used as a simple form of redirection. This kind of redirect is not supported by all browsers and can be confusing to the user. If you need to change the URL of a page as it is shown in search engine results, we recommended that you use a server-side 301 redirect instead. Additionally, W3C's "Techniques and Failures for Web Content Accessibility Guidelines 2.0" lists it as being deprecated.

Written by John Mueller, Webmaster Trends Analyst, Zürich

In writing and maintaining accurate meta tags (e.g., descriptive titles and robots information), you help Google to more accurately crawl, index and return your site in search results. Meta tags provide information to all sorts of clients, such as browsers and search engines. Just keep in mind that each client will likely only interpret the meta tags that it uses, and ignore the rest (although they might be useful for other reasons).

Here's how Google would interpret meta tags of this sample HTML page:

<meta name="description" content="A description of the page">
This tag provides a short description of the page. In some situations this description is used as a part of the snippet shown in the search results. For more information, please see our blog post "Improve snippets with a meta description makeover" and the Help Center article "How do I change my site's title and description?" While the use of a description meta tag is optional and will have no effect on your rankings, a good description can result in a better snippet, which in turn can help to improve the quality and quantity of visitors from our search results.

<title>The title of the page</title>
While technically not a meta tag, this tag is often used together with the "description." The contents of this tag are generally shown as the title in search results (and of course in the user's browser when visiting the page or viewing bookmarks). Some additional information can be found in our blog post "Target visitors or search engines?", especially under "Make good use of page titles."

(X)HTML and Capitalization
Google can read both HTML and XHTML-style meta tags (regardless of the code used on the page). In addition, upper or lower case is generally not important in meta tags -- we treat <TITLE> and <title> equally. The "verify-v1" meta tag is an exception, it's case-sensitive.

revisit-after Sitemap lastmod and changefreq
Occasionally webmasters needlessly include "revisit-after" to encourage a search engine's crawl schedule, however this meta tag is largely ignored. If you want to give search engines information about changes in your pages, use and submit an XML sitemap. In this file you can specify the last-modified date and the change-frequency of the URLs on your site.