The Standard Generalized Markup Language (SGML) is a metalanguage in which one can define markup languages for documents. SGML is a descendant of IBM's Generalized Markup Language (GML), developed in the 1960s by Charles Goldfarb, Edward Mosher and Raymond Lorie (whose surname initials also happen to be GML). SGML should not be confused with the Geography Markup Language (GML) developed by the Open GIS Consortium; cf.
SGML provides a variety of markup syntaxes that can be used for many applications. By changing the SGML Declaration one does not even need to use "angle brackets" although they are the norm, the so-called concrete reference syntax.
SGML was originally designed to enable the sharing of machine-readable documents in large projects in government and the aerospace industry, which have to remain readable for several decades—a very long time in information technology. It has also been used extensively in the printing and publishing industries, but its complexity has prevented its widespread application for small-scale general-purpose use.
SGML syntax example:
typically something like <ITALICS>this</ITALICS>
SGML is an ISO standard: "ISO 8879:1986 Information processing—Text and office systems—Standard Generalized Markup Language (SGML)"
XML is derived from SGML. XML is a profile—a specific subset of SGML, designed to be simpler to parse and process than full SGML. Another markup language originally created as an application of SGML is DocBook, designed for authoring technical documentation. DocBook is now also available as an XML application.
XML is an attempt to simplify SGML for general-purpose applications, such as the Semantic Web. XML has been used for a large number of applications, including notably XHTML, RSS, XML-RPC and SOAP.
HTML, although originally designed independently, was later reformulated (at version 2.0) to be an application of SGML, although there's some debate on whether it ever actually become one.
There are also a number of languages that are related in part to SGML and XML, but, because they cannot be parsed or validated or otherwise processed using standard SGML and XML tools, cannot be considered to be applications of SGML or XML. One example is the Z Format, a language designed for typesetting and documentation.