What are meta tags?

They are vital information about your website inserted into the "head" area of your web pages. Other than part of the title tag (explained below), information in the head area of your web pages is not seen by those viewing your pages in browsers. Instead, meta information in this area is used to communicate information that a human visitor may not be concerned with. Meta tags, for example, can tell a browser what "character set" to use or what are the keywords and purpose of your site is. The first thing a search engine look at to determine the purpose of your website are the meta tags components, mainly the Title Tag, the Keyword Tags and the Description Tags.

Even thought the Meta tag in html is not a required tag when you're creating your web pages and most of the pages don't use the tag at all, the meta tag is used by search engines to allow them to more accurately list your site in their indexes. They are the first and most important set of information major search engines like Google and Yahoo uses to accurately index your site.

You may be able to exert a certain amount of control over how your site is indexed by the use of the meta tags though not all search engines will use this tag, google uses it very extensively. In case of Yahoo you will see that the first set of phrases get more importance than the subsequent ones. Its not a total cure-all therefore, but you will benefit a great deal by putting the tag in, and it may well work well in some cases to your advantage.

What does a meta tag look like?
You should insert the meta tag element at the top of your document, just after the <TITLE> element. It follows the usual form of tags, ie
<META name="something" content="something else">
but note that you don't have to have a </META> at the end of the tag, the way that you do with something like <BOLD> bold </BOLD>. However, make sure that each tag does not include any line breaks, since some search engines get a little bit confused about this.

What can I include in a meta tag?
There are basically four major meta tags that you can use:

<META name="resource-type" content="document">
The only resource type that is currently in use is "document" This is the only tag that you need to put in for indexing purposes, but use of the others is a good idea.

<META name="description" content="a description of your page">

Depending on the search engine, this will be displayed along with the title of your page in an index. "content" could be a word, sentence or even paragraph to describe your page. Keep this reasonably short, concise and to the point. However, don't be so mean with your description that its not an appropriate reflection of the contents!

<META name="keywords" content="a, list, of, keywords">

Choose whatever keywords you think are appropriate, seperated by commas. Remember to include synonyms, americanisms, various spellings and so on. So, if you had a page on cars, you might want to include keywords such as car, cars, vehicles, auto, automobiles and so on.

<META name="distribution" content="one of several">
Content should contain either global, local or iu (for Internal Use). To be perfectly honest, I can't quite get my head around this one; its supposed to list available resources designed to allow the use to find things easily, but I still don't quite get it. My advice is to stick to "global".
Are there optional tags?
Yup, theres a whole bunch of 'em. I've put in a couple of examples below:

<META name="copyright" content="copyright statement"
Pretty obvious what this one's for.
<META HTTP-EQUIV="varname" content="data"
This binds the varname to an HTTP header field. An http server might use this to process a document. This one's a tad more tricky.If you included the following example:
<META HTTP-EQUIV="keywords" content="car,cars"
then, as part of a GET command the server would include the word car and cars in the HTTP response header.
<META HTTP-EQUIV="refresh" content="0; url=homepage.htm"
This can be used in the HEAD section of the index.html file to redirect it to homepage.htm. The figure after content is the time in seconds that the browser waits before moving on.
There are a few others that you can include, such as "revisit-after" and "rating" if you want to be really comprehensive.