Windows Communication Foundation (WCF) was introduced by microsoft as a part of .NET framework 3.0 for building service-oriented applications. Using WCF, we can develop highly secure, reliable applicaitons.
Before WCF was introuduced, for developing distributed applicaitons we need to use different approaches like Web Services, .NET Remoting, MSMQ (Micorsoft message Queuing) and COM+. and WCF was introduced by combing all these features into a single framework to build service oriented applications.
WCF Supports Interoperability, It can communicate with .NET older version applciations(.NET 2.0 web services), WCF Client, Java client, Php client running on windows OS or other operating systems.