WCF = Windows Communication Foundation
A communication-oriented set of APIs and a “runtime” inside .NET to make two (or more) systems talk to one another. It basically replaces ASMX (ASP.NET web services), .NET remoting (object remoting) and a few other communication-related API’s and products in the .NET space.
It can and should be used any time two systems (apps, machines) need to exchange information, basically. It’s the foundation for all “connected systems”.
Your ultimate destination is the MSDN Developer Center for WCF which has a ton of tutorials, articles, sample code, screen casts, videos and much more.