A markup language is mechanism for annotating, or marking up, text in a manner that is syntactically distinguishable from that text. An example of a markup language is the extensible markup language (XML), which codifies a set of rules for encoding documents particularly in a machine-readable form. An XML schema, or XML schema document (XSD), defines the constraints, structure, and content of a corresponding set of XML documents.
XML forms the basis for other languages in which markup language documents can be written, including the web services description language (WSDL), as well as the simple object access protocol (SOAP). The latter is a protocol specification for exchanging structured information over web services using computer networks. A web service is a manner by which two electronic devices can communicate over such networks.