Meta tags appear in the header portion of an HTML page. They supply information about a page but do not affect its appearance. Meta tags used to be the primary method of SEO.

The meta description and keyword tags were especially prominent in this. Many web hosting companies and amateur SEOs still push meta tags as a method of site promotion but due to abuse and other advances in search technology, meta tags no longer hold the same power for SEO. In fact, the MSN Development Team claims they do not use the meta keyword tag at all for determining relevance.

But other search engines are still influenced by meta tags and there are cases where meta tags can help SEO efforts indirectly (even in MSN).

Quick Links: Keywords | Description | Content-Language | Robots | Content-Type

Keywords

  <meta name="keywords" content="keyword1, keyword2, keyword3" />

Search engines originally used the keywords tag to index web pages along with words from the title and document body. Due to spamming this has been radically devalued. It is still a good idea to use synonyms and alternates for your main keywords as well as common misspellings. Make sure the keywords you use are actually relevant to the content in the page. And keep this as brief as possible.

Description

  <meta name="description" content="A short blurb on the page contents." />

The description tag should describe the content of the page, similar to a title. It seems the description tag has almost no bearing on SERP position. However, the meta description has an important role in Google as it often appears in the SERPs. You want your description to be human-friendly to encourage click-thrus.

Content-Language

  <meta http-equiv="content-language" content="en-US" />

This declares the primary language of the document and may influence search engines when they categorize by language. This is especially helpful for non-English web sites.

Robots

  <meta name="robots" content="noindex,nofollow" />

This gives search engine spiders (or bots) specific commands on following links and indexing the page.

• Index – Tells the spider to index the content of the page. This is the default setting and does not need to be specified.
• Noindex – Prevents the spider from indexing the page. The effect this can have on SEO is preventing spiders from indexing a page before it is ready or to avoid duplicate content penalties. The page can not appear in the SERPs if is not indexed.
• Follow – Tells the spider to follow the links going out of a page. This is the default setting and does not need to be specified.
• Nofollow – Prevents the spiders from following the links out of a page. This is useful to SEO when you have links that point to bad neighborhoods and you don’t want your site penalized for it. This is also useful when your page might point to a bad neighborhood, as in the case of an RSS feed where you don’t directly control the links.
• None – tells the spider to ignore the page.
• Noarchive – prevents the spider from archiving the page contents.

The Robots Meta tag was recently updated to create the option of blocking the ODP (Open Directory Project aka DMOZ) description from being used in the SERPs. So far just Google and MSN are supporting this. You an block both search engines from doing this or just Google or just MSN:

• <meta name="robots" content="noodp"> - Blocks both
• <meta name="googlebot" content="noodp"> - Prevents Google from using the ODP description.
• <meta name="msnbot" content="noodp"> - Prevents MSN from using the ODP description.

Content-Type

  <meta http-equiv="content-type" content="text/html; charset=utf-8" />

References the character set. Content-type is recommended to ensure the spiders read your page correctly.

Meta Keywords Tips

As mentioned, abuse has led to the devaluation of meta tags for SEO. To get the most benefit from meta tags do not "keyword stuff" or place keywords that are not relevant to your page content. Your site could get a penalty for this.

Be brief. A meta keywords tag with 50 keywords is not as potent as a keyword tag with 4 words. The model I use is if a web page had 100 points for relevancy and 10 points were from the meta keywords tag, each word can be worth 2 points or one-tenth of a point. This isn’t how it actually works, but I’m sure you understand the concept.

Avoid repeating keywords. If you are optimizing for "red widgets that talk" your keyword list should look like: widgets, red, talk, talking, speak, speaking.

Meta Tags to Avoid

Avoid using meta redirects! Your .htaccess file is a better tool to accomplish this.

Do not use meta refresh! This can prevent the spider from completing the scan of your page and cause confusion.

There are other meta tags but they do not have any influence on SEO that I can determine.