From Jennifer Kyrnin,
Your Guide to Web Design / HTML.
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Meta tags are a lot of things, but basicaly they contain information about the Web page that will not be directly displayed to the person reading the page.

The most common types of meta tags you'll find on the Web are:

• keywords
  Keywords are used by search engines to make the page more easily searchable. You would put words that are used on the page that best describe the page.

  <meta name="keywords" content="antivirus Norton AntiVirus Enterprise edition nav AV">

• description
  The description is used by search engines to provide a short synopsis of the page. You would put a sentence or two that describes what the page is about.

  <meta name="description" content="Norton AntiVirus Enterprise Edition is the most comprehensive antivirus solution available for businesses">

There is also a meta tag that provides a slightly different function. These types of meta tags actually tell the Web server or browser to do something with or to the page. The most common of this type of meta tag is the Refresh tag. It is used to either refresh the current page or move the browser focus to an entirely new page.

Refresh the page to the Web Design home page after 10 seconds:

<meta http-equiv="refresh" content="10;url=http://webdesign.about.com/">

Refresh the page itself after 30 seconds (perhaps to get new information from a spreadsheet or CGI)

<meta http-equiv="refresh" content="30">

One last thing to note: Many Web pages write keyword and description meta tags as "http-equiv" tags. This is incorrect. The correct notation is like I wrote above. For example:

WRONG

<meta http-equiv="keywords" content="antivirus Norton AntiVirus Enterprise edition nav AV">

CORRECT

<meta name="keywords" content="antivirus Norton AntiVirus Enterprise edition nav AV">

Once you understand that meta tags are not meant to be seen by the reader, but rather to be used within the browser itself, it makes them a lot easier to use.