Extensible Markup Language.  XML is a markup language which describe the structure of data.  XML is a simplified form of Standard Generalized Markup Language (SGML). Think of XML as a language that help you understand other languages. 

XML helps with the sharing of information on the Web through identifying and simplifying of language in a document.  Extensible Markup Language (XML) makes it easier to parse data as it is design for wider use to fit various applications.