Question - What is SGML?
Answer -
SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining descriptions of the structure of different types of electronic document.
SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for nearly two decades, and there is a significant body of expertise and software to go with it.
XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but removes all the optional features which made SGML too complex to program for in a Web environment.