SOAP (Simple Object Access Protocol) is the de facto standard Web services communications protocol. In other words, SOAP provides a mechanism that can be used to exchange messages between Web service clients and servers. UDDI (Universal Description, Discovery and Integration) is the de facto standard Web services discovery mechanism. In other words, UDDI provides a mechanism that can be used to find a Web service that meets your requirements and to find information about how to use the service -- usually specified in a WSDL (Web Services Description Language) document. The WSDL document describes what the Web service does (what operations it offers and what messages need to be exchanged), how to use it (what protocols and data encoding systems it uses), and where it's located (its access point).
These three technologies (SOAP, UDDI, and WSDL) form the core of what most people view as the standard Web services infrastructure. Please note that you don't have to use these technologies to implement Web services, but if you want other folks to be able to use your Web services, it helps if you follow these standards.
One point of note: UDDI is itself a Web service. You access UDDI using SOAP, and its interface is described using WSDL. Keep in mind, though, that any type of service can be registered in UDDI. Services registered in UDDI are not required to expose a SOAP interface, nor are they required to have WSDL descriptions.
This was first published in February 2002