WPF is the Microsoft XAML technology for client applications hosted on a Windows operating system.
The current version of WPF is the 4.0, which is delivered with the .NET Framework 4.
WPF is a vectorial UI technology that uses XAML as a markup language to define the UI and uses C# or VB .NET for the presentation logic.
A WPF application can be built with a simple text editor and the C# compiler carried with the NET Framework (it's a tough job, but it can be done); with one of the available versions of Visual Studio 2010 or with Expression Blend which requires both the NET Framework runtime. Visual Studio is for a more development-oriented audience, and Expression Blend is for a more designer-oriented audience.
As you already ...