Web 服务描述语言（Web Services Description Language，WSDL）提供了一种描述 Web 服务（大多使用 SOAP）的简单方法。WSDL 允许您描述利用 SOAP 标准所提供的服务和接口。
比方说，可以创建描述某台服务器上提供的服务的 WSDL 文件，然后把该文件分发给需要这些服务的 Web 服务消费者。通过阅读和解析 WSDL 文件，消费者能够了解到使用这些 Web 服务需要知道的所有信息，包括可以交换的数据类型、参数以及返回的各种错误和其他信息。
再次使用来自 W3C 的例子，可以看到不同远程函数的声明和交换的数据都是通过结构的 XML 定义处理的，如清单 3 所示。
清单 3. 不同远程函数和交换数据的 XML 定义

<?xml version="1.0"?><!-- root element wsdl:definitions defines set of related services --><wsdl:definitions name="EndorsementSearch" targetNamespace="http://namespaces.snowboard-info.com" xmlns:es="http://www.snowboard-info.com/EndorsementSearch.wsdl" xmlns:esxsd="http://schemas.snowboard-info.com/EndorsementSearch.xsd" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"> <!-- wsdl:types encapsulates schema definitions of communication types; here using xsd --> <wsdl:types> <!-- all type declarations are in a chunk of xsd --> <xsd:schema targetNamespace="http://namespaces.snowboard-info.com" xmlns:xsd="http://www.w3.org/1999/XMLSchema"> <!-- xsd definition: GetEndorsingBoarder [manufacturer string, model string] --> <xsd:element name="GetEndorsingBoarder"> <xsd:complexType> <xsd:sequence> <xsd:element name="manufacturer" type="string"/> <xsd:element name="model" type="string"/> </xsd:sequence> </xsd:complexType> </xsd:element> <!-- xsd definition: GetEndorsingBoarderResponse [... endorsingBoarder string ...] --> <xsd:element name="GetEndorsingBoarderResponse"> <xsd:complexType> <xsd:all> <xsd:element name="endorsingBoarder" type="string"/> </xsd:all> </xsd:complexType> </xsd:element> <!-- xsd definition: GetEndorsingBoarderFault [... errorMessage string ...] --> <xsd:element name="GetEndorsingBoarderFault"> <xsd:complexType> <xsd:all> <xsd:element name="errorMessage" type="string"/> </xsd:all> </xsd:complexType> </xsd:element> </xsd:schema> </wsdl:types> <!-- wsdl:message elements describe potential transactions --> <!-- request GetEndorsingBoarderRequest is of type GetEndorsingBoarder --> <wsdl:message name="GetEndorsingBoarderRequest"> <wsdl:part name="body" element="esxsd:GetEndorsingBoarder"/> </wsdl:message> <!-- response GetEndorsingBoarderResponse is of type GetEndorsingBoarderResponse --> <wsdl:message name="GetEndorsingBoarderResponse"> <wsdl:part name="body" element="esxsd:GetEndorsingBoarderResponse"/> </wsdl:message> <!-- wsdl:portType describes messages in an operation --> <wsdl:portType name="GetEndorsingBoarderPortType"> <!-- the value of wsdl:operation eludes me --> <wsdl:operation name="GetEndorsingBoarder"> <wsdl:input message="es:GetEndorsingBoarderRequest"/> <wsdl:output message="es:GetEndorsingBoarderResponse"/> <wsdl:fault message="es:GetEndorsingBoarderFault"/> </wsdl:operation> </wsdl:portType> <!-- wsdl:binding states a serialization protocol for this service --> <wsdl:binding name="EndorsementSearchSoapBinding" type="es:GetEndorsingBoarderPortType"> <!-- leverage off soap:binding document style ...(no wsdl:foo pointing at the soap binding) --> <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/> <!-- semi-opaque container of network transport details classed by soap:binding above ... --> <wsdl:operation name="GetEndorsingBoarder"> <!-- again bind to SOAP? ... --> <soap:operation soapAction="http://www.snowboard-info.com/EndorsementSearch"/> <!-- further specify that the messages in the wsdl:operation "GetEndorsingBoarder" use SOAP? ... --> <wsdl:input> <soap:body use="literal" namespace="http://schemas.snowboard-info.com/EndorsementSearch.xsd"/> </wsdl:input> <wsdl:output> <soap:body use="literal" namespace="http://schemas.snowboard-info.com/EndorsementSearch.xsd"/> </wsdl:output> <wsdl:fault> <soap:body use="literal" namespace="http://schemas.snowboard-info.com/EndorsementSearch.xsd"/> </wsdl:fault> </wsdl:operation> </wsdl:binding> <!-- wsdl:service names a new service "EndorsementSearchService" --> <wsdl:service name="EndorsementSearchService"> <wsdl:documentation>snowboarding-info.com Endorsement Service</wsdl:documentation> <!-- connect it to the binding "EndorsementSearchSoapBinding" above --> <wsdl:port name="GetEndorsingBoarderPort" binding="es:EndorsementSearchSoapBinding"> <!-- give the binding an network address --> <soap:address location="http://www.snowboard-info.com/EndorsementSearch"/> </wsdl:port> </wsdl:service> </wsdl:definitions>
WSDL 声明了消息类型、默认数据类型和内容以及交换的数据结构。
访问服务器上 SOAP 结构需要使用的一切信息都可以在这个 WSDL 中找到。大多数语言和环境都提供一种阅读和解析 WSDL 的机制，以确定可用的函数和数据交换。
WSDL 不仅定义了用于交换信息的 SOAP 接口，通过适当的 WSDL 生成程序，还可用于创建发送请求、生成并格式化响应所需要的代码。
WSDL 和 SOAP 组成了一个强大的远程过程调用系统。