The Web Services Description Language, describes the API a web service exposes to the world. It is usually generated from an interface, rarely written from scratch (unless the programmer likes pain, a lot)

A WSDL document usually has 4 major sections: type definition, message definition (which refers to the types), port description (what input and output message are associated), and a binding definitions (how the ports map into the services methods).

The Google Search WSDL
<?xml version="1.0"?>

<!-- WSDL description of the Google Web APIs.
     The Google Web APIs are in beta release. All interfaces are subject to
     change as we refine and extend our APIs. Please see the terms of use
     for more information. -->

<!-- Revision 2002-08-16 -->

<definitions name="GoogleSearch"
             targetNamespace="urn:GoogleSearch"
             xmlns:typens="urn:GoogleSearch"
             xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
             xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/"
             xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
             xmlns="http://schemas.xmlsoap.org/wsdl/">

  <!-- Types for search - result elements, directory categories -->

  <types>
    <xsd:schema xmlns="http://www.w3.org/2001/XMLSchema" 
                targetNamespace="urn:GoogleSearch">
            
      <xsd:complexType name="GoogleSearchResult">
        <xsd:all>
          <xsd:element name="documentFiltering"           type="xsd:boolean"/>
          <xsd:element name="searchComments"              type="xsd:string"/>
          <xsd:element name="estimatedTotalResultsCount"  type="xsd:int"/>
          <xsd:element name="estimateIsExact"             type="xsd:boolean"/>
          <xsd:element name="resultElements"              type="typens:ResultElementArray"/>
          <xsd:element name="searchQuery"                 type="xsd:string"/>
          <xsd:element name="startIndex"                  type="xsd:int"/>
          <xsd:element name="endIndex"                    type="xsd:int"/>
          <xsd:element name="searchTips"                  type="xsd:string"/>
          <xsd:element name="directoryCategories"         type="typens:DirectoryCategoryArray"/>
          <xsd:element name="searchTime"                  type="xsd:double"/>
        </xsd:all>
      </xsd:complexType>

      <xsd:complexType name="ResultElement">
        <xsd:all>
          <xsd:element name="summary" type="xsd:string"/>
          <xsd:element name="URL" type="xsd:string"/>
          <xsd:element name="snippet" type="xsd:string"/>
          <xsd:element name="title" type="xsd:string"/>
          <xsd:element name="cachedSize" type="xsd:string"/>
          <xsd:element name="relatedInformationPresent" type="xsd:boolean"/>
          <xsd:element name="hostName" type="xsd:string"/>
          <xsd:element name="directoryCategory" type="typens:DirectoryCategory"/>
          <xsd:element name="directoryTitle" type="xsd:string"/>
        </xsd:all>
      </xsd:complexType>
  
      <xsd:complexType name="ResultElementArray">
        <xsd:complexContent>
          <xsd:restriction base="soapenc:Array">
             <xsd:attribute ref="soapenc:arrayType" wsdl:arrayType="typens:ResultElement[]"/>
          </xsd:restriction>
        </xsd:complexContent>
      </xsd:complexType>

      <xsd:complexType name="DirectoryCategoryArray">
        <xsd:complexContent>
          <xsd:restriction base="soapenc:Array">
             <xsd:attribute ref="soapenc:arrayType" wsdl:arrayType="typens:DirectoryCategory[]"/>
          </xsd:restriction>
        </xsd:complexContent>
      </xsd:complexType>

      <xsd:complexType name="DirectoryCategory">
        <xsd:all>
          <xsd:element name="fullViewableName" type="xsd:string"/>
          <xsd:element name="specialEncoding" type="xsd:string"/>
        </xsd:all>
      </xsd:complexType>

    </xsd:schema>
  </types> 

  <!-- Messages for Google Web APIs - cached page, search, spelling. -->
             
  <message name="doGetCachedPage">
    <part name="key"            type="xsd:string"/>
    <part name="url"            type="xsd:string"/>
  </message>

  <message name="doGetCachedPageResponse">
    <part name="return"         type="xsd:base64Binary"/>
  </message>

  <message name="doSpellingSuggestion">
    <part name="key"            type="xsd:string"/>
    <part name="phrase"         type="xsd:string"/>
  </message>

  <message name="doSpellingSuggestionResponse">
    <part name="return"         type="xsd:string"/>
  </message>

  <!-- note, ie and oe are ignored by server; all traffic is UTF-8. -->

  <message name="doGoogleSearch">
    <part name="key"            type="xsd:string"/>
    <part name="q"              type="xsd:string"/>
    <part name="start"          type="xsd:int"/>
    <part name="maxResults"     type="xsd:int"/>
    <part name="filter"         type="xsd:boolean"/>
    <part name="restrict"       type="xsd:string"/>
    <part name="safeSearch"     type="xsd:boolean"/>
    <part name="lr"             type="xsd:string"/>
    <part name="ie"             type="xsd:string"/>
    <part name="oe"             type="xsd:string"/>
  </message>

  <message name="doGoogleSearchResponse">
    <part name="return"         type="typens:GoogleSearchResult"/>           
  </message>

  <!-- Port for Google Web APIs, "GoogleSearch" -->

  <portType name="GoogleSearchPort">

    <operation name="doGetCachedPage">
      <input message="typens:doGetCachedPage"/>
      <output message="typens:doGetCachedPageResponse"/>
    </operation>

    <operation name="doSpellingSuggestion">
      <input message="typens:doSpellingSuggestion"/>
      <output message="typens:doSpellingSuggestionResponse"/>
    </operation>

    <operation name="doGoogleSearch">
      <input message="typens:doGoogleSearch"/>
      <output message="typens:doGoogleSearchResponse"/>
    </operation>

  </portType>


  <!-- Binding for Google Web APIs - RPC, SOAP over HTTP -->

  <binding name="GoogleSearchBinding" type="typens:GoogleSearchPort">
    <soap:binding style="rpc"
                  transport="http://schemas.xmlsoap.org/soap/http"/>

    <operation name="doGetCachedPage">
      <soap:operation soapAction="urn:GoogleSearchAction"/>
      <input>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </input>
      <output>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </output>
    </operation>

    <operation name="doSpellingSuggestion">
      <soap:operation soapAction="urn:GoogleSearchAction"/>
      <input>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </input>
      <output>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </output>
    </operation>

    <operation name="doGoogleSearch">
      <soap:operation soapAction="urn:GoogleSearchAction"/>
      <input>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </input>
      <output>
        <soap:body use="encoded"
                   namespace="urn:GoogleSearch"
                   encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </output>
    </operation>
  </binding>

  <!-- Endpoint for Google Web APIs -->
  <service name="GoogleSearchService">
    <port name="GoogleSearchPort" binding="typens:GoogleSearchBinding">
      <soap:address location="http://api.google.com/search/beta2"/>
    </port>
  </service>

</definitions>