Web Services support both synchronous and asynchronous communication between the client and the server that hosts the Web Service.  Under synchronous communication, the client sends a request to the server and waits for the response.  This prevents the client from performing other operations while waiting for the results.  On the other hand, in asynchronous communication, the client continues processing other tasks as it waits for a response.  The client responds to the result of the service request when it becomes available.