In WCF, all services expose contracts. The contract is a platform-neutral and standard way of describing what the service does. WCF defines four types of contracts:
Service contracts
Describe which operations the client can perform on the service.
Data contracts
Define which data types are passed to and from the service. WCF defines implicit contracts for built-in types such as int and string, but you can easily define explicit opt-in data contracts for custom types.
Fault contracts
Define which errors are raised by the service and how the service handles and propagates errors to its clients.
Message contracts
Allow the service to interact directly with messages. Message contracts can be typed or untyped and are useful in interoperability cases when another party has already dictated some explicit (typically proprietary) message format. This, however, is by no means the usual case for common WCF applications. Unless you are required to leverage the flexibility, power, and extensibility of message contracts, you should avoid them, as they add no value, but do add complexity. In many cases, the desire to use message contracts indicates a need for a custom application context, which you can address using custom headers .