SGML is a much broader language which is used to define particular markup languages for particular purposes. HTML is just a specific application of SGML. You can learn more about SGML, and the rationale behind HTML, by reading A Gentle Introduction to SGML (URL is <URL:http://etext.virginia.edu/bin/tei-tocs?div=DIV1&id=SG> ), a document provided by the Text Encoding Initiative.