From LQWiki

SGML -- The Standard Generalized Markup Language. Not really a language, but a meta-language, it defines how you can define your own markup language using a DTD (Document Type Definition). Most of today's markup languages are defined using SGML. HTML, the most famous example, is a very simple, very small subset of SGML. XML, the eXtensible Markup Language, is a modern, fully functional, robust approach. XML is software-readable, and various dialects of it are used for everything from MS Word's latest format to build files for Ant, the Java-based build-management tool. XHTML is HTML written using XML rules for well-formedness. It aims to become the new Web standard in accessibility.