Code-First and Contract-First Development Style

Marking a class or interface with ServiceContract and one or more of its methods with OperationContract also allows automatically generating service contract definitions in WSDL. Accordingly, the externally visible definition of every WCF service contract can be accessed as a standard WSDL document specifying the operations in that contract. This style of development, commonly called code-first, allows creating a standard interface definition directly from types defined in programming languages such as C# or Visual Basic.

An alternative approach is contract-first development, an option that WCF also supports. In this case, a developer typically starts with a WSDL document describing the interface (i.e., the contract) that a service class must implement. Using a tool called svcutil, the developer can generate a skeleton service class directly from a WSDL definition.

copied from MSDN for my own reference.

http://msdn.microsoft.com/library/ee958158.aspx