WPF stands for Windows Presentation Foundation.
Microsoft describes WPF as follows:
Windows Presentation Foundation (WPF) provides developers with a unified programming model for building rich Windows smart client user experiences that incorporate UI, media, and documents. 
Windows Presentation Foundation (WPF) is a next-generation presentation system for building Windows client applications with visually stunning user experiences. With WPF, you can create a wide range of both standalone and browser-hosted applications. 
WPF is part of .NET Framework and is only available currently on Windows platforms that can run .NET Framework 3 or higher.
WPF is not just for developers. It is designed to allow developers and designers to work together. Developers usually interface with WPF through Visual Studio, while designer interface with WPF using Expression Blend.