SGML or Standard Generalized Markup Language is a standard for organizing and tagging elements of a document. SGML is not in itself a document language, but a description of how to specify one. It specifies the rules for tagging elements. These tags can then be interpreted to format elements in different ways. SGML was developed and standardized by the International Organization for Standards (ISO) in 1986.
SGML is based on the idea that documents have structural and other semantic elements that can be described without reference to how such elements should be displayed. The actual display of such a document may vary, depending on the output medium and style preferences.
SGML is used widely to manage large documents that are subject to frequent revisions and need to be printed in different formats. It is not so widely used on personal computers though as it’s a large and complex system. However, the growth of Internet, and especially the World Wide Web, is creating renewed interest in SGML because Hypertext Markup Language (HTML), is an example of an SGML-based language and the World Wide Web uses HTML, which is one way of defining and interpreting tags according to SGML rules.