What is a Meta Tag and SEO?

First of all, don't let me mislead you, meta tags are not the "magic bullet" that will skyrocket your site to the top of all search page listings. They are a tool that will help improve your standings in search engines that use them. Use them with other marketing strategies to garner more page views.
Another thing to remember: most search engines look at the body of text on your pages, as well as the page title. They take this information as higher relevance than any meta tags. So, be sure to always have a relevant (TITLE) on your pages, and relevant content in the body of the page. This will improve your rankings more than just meta tags alone.
What is a Meta Tag?
A meta tag is a hidden tag that lives in the (HEAD) of an HTML document. It is used to supply additional information about the HTML document. The meta tag has three possible attributescontent,http-equiv, andname. Meta tags always provide information in a name/value pair. Thenameandhttp-equivattributes provide the name information and thecontentprovides the value information. Meta tags do not have a closing tag.
content
This attribute will always be found in a well formed meta tag. It provides the value information in the name/value pair. It can be any valid string, which you should enclose in quotes.
name
This is the name portion in the name value pair. You can use any name that you would like or that might be useful to you. Some common names are:

• keywords - words that identify what the page is about, usually used in search engines
  (meta name="keywords" content="HTML, HTML help, meta tags, promotion, web sites")
  
• description - a short description of the page
  (meta name="description" content="Boost your marketing strategy with meta tags")
  
• author - the author's name and possibly email address
  (meta name="author" content="Jennifer Kyrnin")
  
• robots - to allow or disallow indexing by robots
  (meta name="robots" content="noindex")
  
• copyright - the copyright date of the page
  (meta name="copyright" content="August 1999")

http-equiv
This attribute is also a name for the name/value pair, but it is used by the server to include that name/value pair in the MIME document header passed to the Web browser before sending the actual HTML document. Some common http-equiv types are:

• charset - defines the character set used in the page
  (meta http-equiv="charset" content="iso-8859-1")
  
• expires - when the document will be out of date
  (meta http-equiv="expires" content="31 Dec 99")
  
• refresh - sets the number of seconds to reload the page or reload to a new page
  Reload the page every 10 seconds:
  (meta http-equiv="refresh" content="10")
  Reload to a new page after 10 seconds:
  (meta http-equiv="refresh" content="10;url=http://webdesign.about.com/")

How to Use a Meta Tag
Meta tags are included in the (HEAD) of an HTML document. If you are using meta tags to improve your standing in search engines, then you should focus on your description and keywords.
The description tag
Use the description tag to describe what your page is about. Engines that use it will supply the content of this tag when displaying a list of links. For example, if you do a search on About.com, you will see the description listed on the search results page.
The keywords tag
Keywords help search engines to categorize your site, and to allow people to find your pages more quickly. However, most search engines have limits as to how many meta keywords are viewed. It is a good idea to review your keywords and make sure that they are as concise and specific as possible.

Note: Replace "(" to "<" and ")" to ">"