What is the difference between WCF and WPF?

WCF = Windows COMMUNICATION Foundation

WPF = Windows PRESENTATION Foundation.

WCF deals with communication (in simple terms - sending and receiving data as well as formatting and serialization involved), WPF deals with presentation (UI)

The quick answer is: Windows Presentation Foundation (WPF) is basically a way of displaying user interface. (see this)

Windows Communication Foundation (WCF) is a framework for creating service oriented applications. (see this)

As for which one you should use, it depends on your requirement. Usually an application written in WPF, ASP.NET..etc called the WCF service to do some processing at the server-side and the service returns the result to the application that called it.

Windows Presentation Foundation (WPF)

Next-Generation User Experiences. The Windows Presentation Foundation, WPF, provides a unified framework for building applications and high-fidelity experiences in Windows Vista that blend application UI, documents, and media content. WPF offers developers 2D and 3D graphics support, hardware-accelerated effects, scalability to different form factors, interactive data visualization, and superior content readability.

Windows Communication Foundation (WCF)

Windows Communication Foundation (WCF) is Microsoft’s unified programming model for building service-oriented applications. It enables developers to build secure, reliable, transacted solutions that integrate across platforms and interoperate with existing investments.