Meta tags are sometimes considered a bit mystical. Possibly because of their name (metaphysical?). And also because they don't show up in your browser. They have no influence on the document structure or appearance. So what are they really for?
First let's clarify the meta concept. You can have a conversation with someone. At some point during that, you start talking about the conversation. Things like: I think we are having a really good talk. Or: This talk isn't really going anywhere, is it? Then you are having a meta conversation, a talk ABOUT the talk.
Meta tags contain information ABOUT the document. They can be used to transfer information about its contents, who the author is, keywords for classification and indexing. But also about how long the document is valid, if it should be cached by a browser or not. And even how long it should be viewed before it is reloaded or another document is loaded (client pull).
This meta information is used by many search engines. In search engine listings you see a title and a short description. This description can be defined with meta tags. The keywords are put in the search engine database for looking up the document. So choosing the right keywords and description may be a decisive factor for others to find your page.
<META NAME="author" CONTENT="Gerben Hoekstra">
The author is specified with this meta tag. A meta tag should always reside between the HEAD tags of a web page. Most have a NAME attribute that define the purpose, and a CONTENT attribute that defines the actual contents. The meta tag above tells this document was written by me.
<META NAME="description" CONTENT="Meta tags are used to convey information about web pages. This page is part of a HTML tutorial on WEBalley.">
The description meta tag is used to give a short description of the contents of the document. Search engines often use this description in their listings. Try to keep this description short, concise, unabmiguous. Some search engines use only the first 10 or 25 words, and chop off the remaining text. You can make it longer some engines will read up to a 1000 characters. Just make sure the first sentence contains the info you need.
<META NAME="keywords" CONTENT="meta tag, html, webpage, www, homepage, tutorial, beginner">
This tag specifies keywords a search engine visitor might use to find your document. Use only keywords that actually have something to do with your document. Repetition of keywords may improve the score of your document in a search. But some engines consider this spamming and drop your document at the bottom of their listings, or decide not to show it at all. Some engines read up to 1000 characters or more.
<META HTTP-EQUIV="Refresh" CONTENT="10;URL=nextpage.html">
You can make a document refresh itself after a set time, or load another page. This meta tag has no NAME attribute. Instead there is a HTTP-EQUIV attribute. The refresh value is used for client pull. The content is specified with the time in seconds and the URL of the page to be loaded. Take great care in the spelling (semicolon, URL, =), otherwise it won't work.
It probably wouldn't be wise to use this on important pages of your site. Sometimes this client pull is used to spam search engines, with several different pages leading to one home page. You may again end up at the bottom of the listing.
When you are using frames on your site, meta tags are actually the only place to put information about the site. There is no body in the frameset page, search engines won't find anyting to review or index. So you'd probably better make sure you use the meta tags here.
There are a lot of other meta tags like these, dozens actually. These are just the most used ones. Others are less used, less important, and often not supported by browsers or web servers. So I would advise against using them. I think you should however always use the author, description and keyword meta tags, in every document on your site.
<TITLE>Page title here</TITLE>
The title tag is not a meta tag. But I want to mention it here, because it is as important as your meta tags, as far as promotion is concerned. The title is the first thing that gets listed in a search engine. Some add the description from the meta tags, some don't, some let you list titles only. Some treat the title as the most decisive factor in indexing the page. So be sure to use a title that tells something about your site, about what the visitor can expect to find there. Not just a name, or "home page" or even "Page title here" (duh), as I am seeing all too often.
I created two utilities to help you create and use meta tags properly. MetaMake generates meta tags for your page. MetaCheck checks if meta tags are used correctly.