
 SGML (Standard Generalised Markup Language):  is a standard for describing markup languages.
 DTD (Document Type Definition):  is the formal specification of a markup language, written using SGML.
 HTML (HyperText Markup Language):  is an SGML DTD in practical terms. HTML is a collection of platform independent styles (indicated by markup tags). It defines the various components of a World Wide Web document.
