WSDL Message Element

• The <message> element describes the data being exchanged between the Web service providers and consumers.
• Each Web Service has two messages: input and output.
• The input describes the parameters for the Web Service and the output describes the return data from the Web Service.
• Each message contains zero or more <part> parameters, one for each parameter of the Web Service's function.
• Each <part> parameter associates with a concrete type defined in the <types> container element.
  

Lets take a piece of code from the Example Session:

<message name="SayHelloRequest">
      <part name="firstName" type="xsd:string"/>
   </message>
   <message name="SayHelloResponse">
      <part name="greeting" type="xsd:string"/>
   </message>

Here, two message elements are defined. The first represents a request message SayHelloRequest, and the second represents a response message SayHelloResponse.

Each of these messages contains a single part element. For the request, the part specifies the function parameters; in this case, we specify a single firstName parameter. For the response, the part specifies the function return values; in this case, we specify a single greeting return value.



References
Wikipedia for WSDL