Definition from MSDN: WCF is a set of .NET technologies for building and running connected systems. It is a new communications infrastructure built around the Web services architecture. Advanced Web services support in WCF provides secure, reliable, and transacted messaging along with interoperability. The service-oriented programming model of WCF is built on the Microsoft .NET Framework and simplifies development of connected systems. It unifies a broad array of distributed systems capabilities in a composable and extensible architecture, spanning transports, security systems, messaging patterns, encodings, network topologies, and hosting models.