Explain how to use the Begin and End methods on a Web Service to make an asynchronous method call.

Every public Web method on a Web Service can be called either synchronously or asynchronously. To make an asynchronous call to a Web method, you call the method named Begin, where is the name of the method. This method requires a delegate to an appropriate callback method and returns a value of IAsyncResult. This value is returned as a parameter in the callback method. To retrieve the data returned by the Web method, call End, supplying a reference to the IAsyncResult returned by Begin. This will allow you to retrieve the actual data returned by the Web method.