Standard Generalized Markup Language SGML (Standard Generalized Markup Language) is used to present the highest level of standard hypertext format is a markup language to define the language without even having to use defined for <> in a conventional manner. Due to the complexity of SGML, making it difficult to spread.
While SGML is an ISO standard: "ISO 8879:1986 Information processing - Text and office systems - Standard Generalized Markup Language (SGML)"
SGML has a very strong adaptability, it is precisely because of the same reason, resulting in small-scale applications difficult to spread.
HTML and XML are derived from the same SGML: XML is considered a subset of SGML, and HTML is an application of SGML.
XML produced is to simplify SGML, so for more general purposes, such as the Semantic Web. It has been applied to a large number of occasions, more notably XHTML, RSS, XML-RPC and SOAP.