Windows Presentation Foundation(WPF) is a computer-software graphical subsystem for rendering user interfaces in Windows-based applications.So it mainly used for Devloping window based application as well as Web based application.
It is a part of dot net framework .WPF attempts to provide a consistent programming model for building applications and provides a separation between the user interface and the business logic.

WPF based on XAML(Extensible Application Markup Language)WPF aims to unify a number of common user interface elements, such as 2D/3D rendering, fixed and adaptive documents, typography, vector graphics, runtime animation, and pre-rendered media. These elements can then be linked and manipulated based on various events, user interactions, and data bindings.

In technical or development level implementation, it also has the huge change as it is using XAML oriented structures where GUI objects,attributes are presented with XML type tag,attributes etc. It doesn't requires C# based code for the creation/initialization of the graphical user interface. But that doesn't mean either that we won't be able to use c# to create/manipulate controls, of course we will be able to do that.Also, Silverlight utilizes wpf for embedded web controls.