“Conceived notionally in the 1960s - 1970s, the Standard Generalized Markup Language (SGML, ISO 8879:1986) gave birth to a profile/subset called the Extensible Markup Language (XML), published as a W3C Recommendation in 1998. Depending upon your perspective and requirements, the differences between SGML and XML are inconsequential or immense. SGML is more customizable (thus flexible and more "powerful") at the expense of being (much) more expensive to implement. In an SGML language you could say <COVER PAGES>, whereas in XML this construct could not be DTD valid.” (OASIS SGML Page, retrieved Aug 2009.
According to Wikipedia's SGML article, the The Standard Generalized Markup Language (ISO 8879:1986 SGML) is an ISO-standard technology for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup:
An example of a simple SMGL application is HTML
See also XML the more popular subset of SGML.