What is SGML?
SGML = Standard Generalized ML
A SGML document carries with it a grammar called a Document Type Definition (DTD). The DTD defines the tags and the meaning of those tags
Presentation is governed by a style sheet written in the Document Style Semantics and Specification Language (DSSSL)
Note that HTML is a fixed SGML application, a hard-wired set of about 70 tags and 50 attributes, and does not need to have a DTD.