- The message parameter in the WSDL definition is used to define the different data elements for each operation performed by the web service. So in the example above, we have 2 messages which can be exchanged between the web service and the client application, one is the "TutorialRequest", and the other is the "TutorialResponse" operation. The TutorialRequest contains an element called "TutorialID" which is of the type string. Similarly, the TutorialResponse operation contains an element called "TutorialName" which is also a type string.

Unlike traditional client/server models, such as a Web server/Web page system, Web services do not provide the user with a GUI. Web services instead share business logic, data and processes through a programmatic interface across a network. The applications interface, not the users. Developers can then add the Web service to a GUI (such as a Web page or an executable program) to offer specific functionality to users. 

Exposing Business Functionality on the network - A web service is a unit of managed code that provides some sort of functionality to client applications or end users. This functionality can be invoked over the HTTP protocol which means that it can also be invoked over the internet. Nowadays all applications are on the internet which makes the purpose of Web services more useful. That means the web service can be anywhere on the internet and provide the necessary functionality as required.

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A web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the web service in a manner prescribed by its description using SOAP-messages, typically conveyed using HTTP with an XML serialization in conjunction with other web-related standards.
"All the standard Web Services works using following components:" ... the problem with this answer is that it suggests that web services do or must operate within the SOAP/WSDL mechanism. This is inaccurate. Representational State Transfer (REST) is another very popular paradigm for implementing web services. Some major websites have deprecated their SOAP services in favor of REST web services. – scottb Jul 7 '15 at 15:50