The eXtensible Markup Language (XML) is a meta-language - language for defining other languages. The XML is used for two different purposes: document-oriented and data-oriented applications.
Document-oriented markup languages like XHTML and DocBook are focused on the format and presentation of literature. They describe how books, magazines, and business and scientific papers are organized, and how they look.
Data-oriented markup languages focus on how data is typed and organized; they declare a schema for storing and exchanging data between software applications.