The Extensible Markup Language (XML) is a subset of the Standard Generalised Markup Language (SGML), the standard for creating markup documents. Unlike HTML, another markup language based on SGML, XML allows you to describe data and its structure rather than display it. XML is not a replacement for HTML, but compliments it by allowing the author to describe their own tags. XML is extensible as the author can create an unlimited number of tags. There are no predefined tags in XML.

All XML documents should start with a processing instruction to inform the XML parser of the version of XML being used.

<?xml version="1.0" ?>

XML is incredibly easy to learn! It's just a case of defining the structure of your data, and creating some rules for the data in the form of a Document Type Definition. The usefulness of XML comes from its intrinsic property of separating the data from the presentation.