XML is part of the class metalanguages, it is actually a subset of SGML. Okay, so what is the SGML (Standard Generalized Markup Language)? It is a language that allows you to create structured and modular documents. Concretely this means that you can create a document from data as diverse as sound, data from a database or text and images. SGML has been defined for a wide range of applications and must be kept on a very long term. This language is quite extensive and complex, the XML was created to simplify SGML.