WSDL - Web Service Description Language

WSDL is a language based on XML. WSDL is a a layer of understanding between web services and the calling clients. it provides

a machine-readable description of how the service can be called,
what parameters it expects
and what data structures it returns.

It thus serves a roughly similar purpose as a Method signature in a programming language.

Objects in WSDL 1.1 / WSDL 2.0

WSDL 1.1 Term	WSDL 2.0 Term	Description
Service	Service	Contains a set of system functions that have been exposed to the Web-based protocols.
Port	Endpoint	Defines the address or connection point to a Web service. It is typically represented by a simple HTTP URL string.
Binding	Binding	Specifies the interface and defines the SOAP binding style (RPC/Document) and transport (SOAP Protocol). The binding section also defines the operations.
PortType	Interface	Defines a Web service, the operations that can be performed, and the messages that are used to perform the operation.
Operation	Operation	Defines the SOAP actions and the way the message is encoded, for example, "literal." An operation is like a method or function call in a traditional programming language.
Message	n/a	Typically, a message corresponds to an operation. The message contains the information needed to perform the operation. Each message is made up of one or more logical parts. Each part is associated with a message-typing attribute. The message name attribute provides a unique name among all messages. The part name attribute provides a unique name among all the parts of the enclosing message. Parts are a description of the logical content of a message. In RPC binding, a binding may reference the name of a part in order to specify binding-specific information about the part. A part may represent a parameter in the message; the bindings define the actual meaning of the part. Messages were removed in WSDL 2.0, in which XML schema types for defining bodies of inputs, outputs and faults are referred to simply and directly.
Types	Types	Describes the data. XML Schema is used (inline or referenced) for this purpose.

WSDL Example

This is a simplified fraction of a WSDL document:

<message name="getTermRequest">   <part name="term" type="xs:string"/> </message> <message name="getTermResponse">   <part name="value" type="xs:string"/> </message> <portType name="glossaryTerms">   <operation name="getTerm">     <input message="getTermRequest"/>     <output message="getTermResponse"/>   </operation> </portType>

In this example the <portType> element defines "glossaryTerms" as the name of a port, and "getTerm" as the name of an operation.
The "getTerm" operation has an input message called "getTermRequest" and an output message called "getTermResponse".
The <message> elements define the parts of each message and the associated data types.
Compared to traditional programming, glossaryTerms is a function library, "getTerm" is a function with "getTermRequest" as the input parameter, and getTermResponse as the return parameter.

Binding to SOAP

A request-response operation example:

<message name="getTermRequest">   <part name="term" type="xs:string"/> </message> <message name="getTermResponse">   <part name="value" type="xs:string"/> </message> <portType name="glossaryTerms">   <operation name="getTerm">     <input message="getTermRequest"/>     <output message="getTermResponse"/>   </operation> </portType> <binding type="glossaryTerms" name="b1">    <soap:binding style="document"    transport="http://schemas.xmlsoap.org/soap/http" />    <operation>      <soap:operation soapAction="http://example.com/getTerm"/>      <input><soap:body use="literal"/></input>      <output><soap:body use="literal"/></output>   </operation> </binding>

The binding element has two attributes - name and type.
The name attribute (you can use any name you want) defines the name of the binding, and the type attribute points to the port for the binding, in this case the "glossaryTerms" port.
The soap:binding element has two attributes - style and transport.
The style attribute can be "rpc" or "document". In this case we use document. The transport attribute defines the SOAP protocol to use. In this case we use HTTP.
The operation element defines each operation that the port exposes.
For each operation the corresponding SOAP action has to be defined. You must also specify how the input and output are encoded. In this case we use "literal"