The Standard Generalized Markup Language is an ISO-standard technology for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup: Generalized markup is based on two novel postulates: Markup should be declarative: it should describe a document’s structure and other attributes, rather than specify the processing to be performed on it. Declarative markup is less likely to conflict with unforeseen future processing needs and techniques.

SGML was powerful and used for complex enterprise publishing and information management applications, but it was difficult and expensive to deploy. 

W3C’s HTML and XML standards were based on SGML. XML was published in 1998 both as a corrective to the complexity of SGML, and to provide much-needed flexibility to HTML. 

For a detailed, non-technical explanation of SGML and its value for business applications see Gilbane Report Vol 1, Num 2 – SGML Open – Why SGML & Why a Consortium