Web services are an important new kind of application. A Web service is a piece of code that can be discovered, described, and accessed using XML. There is a great deal of activity in this space, but the three main XML standards for Web services are:

• SOAP: Originally the Simple Object Access Protocol, SOAP defines an XML document format that describes how to invoke a method of a remote piece of code. My application creates an XML document that describes the method I want to invoke, passing it any necessary parameters, and then it sends that XML document across a network to that piece of code. The code receives the XML document, interprets it, invokes the method I requested, then sends back an XML document that describes the results. Version 1.1 of the SOAP spec is at w3.org/TR/SOAP/. Visit w3.org/TR/ to see all of the W3C's SOAP-related activities.
• WSDL: The Web Services Description Language is an XML vocabulary that describes a Web service. It's possible to write a piece of code that takes a WSDL document and invokes a Web service it's never seen before. The information in the WSDL file defines the name of the Web service, the names of its methods, the arguments to those methods, and other details. You can find the latest WSDL spec at w3.org/TR/wsdl (no closing slash).
• UDDI: The Universal Description, Discovery, and Integration protocol defines a SOAP interface to a registry of Web services. If you have a piece of code that you'd like to deploy as a Web service, the UDDI spec defines how to add the description of your service to the registry. If you're looking for a piece of code that provides a certain function, the UDDI spec defines how to query the registry to find what you want. The source of all things UDDI is uddi.org.