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SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining markup to describe the structure of different types of electronic document. There is an SGML FAQ from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.html; and Robin Cover's SGML Web pages are at http://www.oasis-open.org/cover/general.html. For a brief summary and list of online and print resources, see http://wiht.link/SGML-intro. For a little light relief, try Joe English's ‘Not the SGML FAQ’ at http://www.flightlab.com/~joe/sgml/faq-not.txt.
SGML is very large, powerful, and complex. It was in heavy industrial and commercial use for nearly two decades (and still is, in some places), and there is a significant body of expertise and software that grew up with it, most of which is now available for XML.
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.