You can use meta tags to specify information about a HTML document that's not considered document content or to send special instructions to the browser. Sample list of frequently used meta tags:
"keywords" - assign a set of keywords to the document.
 
"description" - specify a summarized description of the document.
 
"Content-Type" - specify character encoding information.
 
"refresh" - redirect visitors to another page or reload the current document.
 
"PICS-Label" - rate content.
 
"robots" - instruct Web robots how to index the current document.
 
Yes, there are many other, mostly proprietary, meta tags used by different software. However, don't hurry up to add all the meta tags you can get your hands on to your pages. This is because only a handful of the meta tags are considered "standard" or atleast recognized by a majority of the software.
 
If you're looking for the definition of a meta tag you've heard about or just curious about what else is out there...