WCF stands for Windows Communication Foundation.
MSDN is a little dry, but Wikipedia expresses it well:
WCF is the part of the .NET Framework dedicated to communications.
In my case, this has meant a richer alternative to plain old Web Services, with a choice between communication layers.
Edit
15 Seconds also has a great primer on WCF:
WCF is a unified programming model that combines the best of breed features from XML Web Services, .NET Remoting, MSMQ, and COM+ into an integrated platform that is completely based on a set of open industry standards.
