An html tag you can stuff in the head of your document. It is designed to pass on useful information to your browser about the document as a whole (rather than actual content, by means of string fields, e.g. <META name="Author" content="Haggai Scolnicov">. META http-equiv is used to tell the browser to do certain things involving http.
Also used to stuff in information aimed at search engine bots and crawlers to help them figure out the structure of a complete site, languages, and other, well, "meta" information.
People don't use this nearly enough in their HTML (prboably because they generated it with some awful html editor), and browsers don't really do anything to make this important info available, if and when it is there, except when it controls browsing behaviour (e.g. meta http-equiv). One omission leads to the other, and vice versa. It's a vicious circle.
See http://www.w3.org/TR/html4/struct/global.html#h-7.4.4 for the for what the Standard says.