SGML is the Standard Generalized Markup Language, a tool for repsenting the abstract structure of a document instead of its visual appearance. The theory is that you can use one piece of information for many purposes by writing programs to format it.
SGML is baroque in the extreme, and few people can implement it correctly. XML is the same thing, but much simpler and far more trendy. HTML is an example of an SGML application. See also DTD, HyTime and DSSSL.