There are three types of message exchange pattern in WCF:
1. One-Way,
2. Request-Reply,
3. Duplex .
Request-Reply is the Default one.
In One Way message exchange pattern , WCF does not need to send reply back to client application and client application does not wait for the reply even.
In Request-Reply, Client application sends message to Service application and waits for the reponse from the service application.
In Duplex communication, client application or service application can start the communication and expect notification/ acknowledgement from their counter part.