DemoServiceClient is the proxy generated by the Visual Studio IDE when you are adding the service reference.
Suppose, you have a WCF service named "MyService" and you added the service reference to your project, it will generate the proxy class in that project to give you easy access to the service contract.
You can then call the methods inside the class i.e. the service call by using this:
DemoServiceClient webService = new DemoServiceClient();<br />
webService.InsertDataAsync(TestItem1TxtBox.Text, TestItem2TxtBox.Text); // assume this is the service call