See definition for Extensible Markup Language (XML).
XML Schema (XSD)
The World Wide Web Consortium (W3C) standard language used for creating XML Schema documents. The XML Schema contains two parts: a set of predefined types (for example, string, dateTime, decimal), and an XML language for defining new types (for example, complexType, minOccurs, element).
XML Schema Document
A specification that describes the complex types used in a Web method and thereby enables interoperability between clients and Web services built on different platforms by adhering to a common type system, as defined by the W3C.
XML Web services
Units of application logic providing data and services to other applications. Applications access XML Web services using standard Web protocols and data formats such as HTTP, XML, and SOAP, independent of how each XML Web service is implemented. XML Web services combine the best aspects of both component-based development and the Web, and they are a cornerstone of the Microsoft .NET programming model.