The official HTML RFC1866 is particularly unhelpful on this tag, describing it as "Associated meta-information".
One of the most useful versions of the META tag should be HTTP-EQUIV. Whenever a web server serves out a file, it first adds a few header lines, including, for example, a line telling the browser what sort (mime type) of file is coming. It is these header lines that tell a browser how to deal with the file it receives. (If for example a server hasn't been told to treat files with an htm extension (and not just html extensions) as being HTML files, your browser will read them as text files and you'll see all the tags!). In principle, the HTTP-EQUIV attribute should either cause the server to add a header line or cause the browser to read it as if a header line had been added. In practice I'm not sure whether it does and the specification (para 5.25) says that the method by which meta-information is extracted is "unspecified and not mandatory".
Another use is offered by the
various search engines. Both Alta Vista and Infoseek suggest the use of
tags for keyword searching and descriptions rather than their automatic
indexing. Thus you might include in the head section of your page:
The Netscape browser also recognises an HTTP-EQUIV function of REFRESH,
which can be used to load a new page a particular time after loading
the first page or (if the URL argument is a sound file) to play music
once the page is loaded. If for example you have an introduction page at
http://www1.url/intro.html and a second page at
http://www2.url/page_2.html, you might include in the head section of the
page at http://www1.url/intro.html a line:
For more information on the META tag see