Basic and Advance XML Programming Question:

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. There is an SGML FAQ from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.htmlFAQ; and Robin Cover's SGML Web pages are at http://www.oasis-open.org/cover/general.html. For a little light relief, try Joe English's ?Not the SGML FAQ? at http://www.flightlab.com/~joe/sgml/faq-not.txtFAQ.
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.