Extensible Markup Language (XML), a subset of SGML, defines a syntax that lets you create markup languages to specify information structures.
Information structures define the type of information, for example, subscriber name or address, not how the information looks (bold, italic, and so on).
External processes can manipulate these information structures and publish them in a variety of formats.
Text markup language designed to enable the use of SGML on the World Wide Web.
XML allows you to define your own customized markup language.
XML tags are not predefined. You must define your own tags.
XML uses a Document Type Definition (DTD) or an XML Schema to describe the data. XML is not a replacement for HTML.
Actually, XML and HTML are complimentary to each other.