XML stands for eXtensible Markup Language. XML was designed to facilitate the sharing of data across different systems, particularly systems connected via the Internet. XML is a "meta" language because it is used for defining markup languages. XML is a subset of the Standard Generalized Markup Language (SGML). A markup language defined using SGML or XML has a specific vocabulary (labels for elements and attributes) and a declared syntax (grammar defining the hierarchy and other features).
XML is the basic building block of Web services. All the Web services technologies recommended by the WS-I Basic Profile 1.0 are built on XML and he XML Schema Language.