WPF (Windows Presentation Foundation) is a graphical subsystem in the .NET framework that provides a way to build visually appealing user interfaces in Windows desktop applications. It is a technology for building visually stunning and interactive user experiences for Windows desktop applications.

WPF uses XAML (eXtensible Application Markup Language) to define the layout, visual appearance, and behavior of the user interface elements. XAML is a markup language that allows developers to separate the logic and presentation of their applications.

Some key features of WPF include:

1. Rich User Interface: WPF provides a wide range of controls, layouts, and styling options to create visually appealing and interactive user interfaces. It supports animations, 3D graphics, media playback, and vector-based graphics.

2. Data Binding: WPF supports powerful data binding capabilities, allowing developers to easily bind the UI controls to data sources. This enables automatic synchronization of data between the UI and the underlying data source.

3. Separation of UI and Logic: With XAML, developers can define the visual appearance and layout of the UI separately from the code that controls its behavior. This promotes a clear separation of concerns and allows designers and developers to work on different aspects of the application without interfering with each other.

4. Resources and Styles: WPF provides a flexible and powerful mechanism for defining reusable resources and styles. This allows for consistent styling across the application and makes it easy to change the appearance of controls globally.

5. Integration with .NET: WPF is built on top of the .NET framework, which means it can take advantage of many other features and libraries provided by .NET. It supports data access, networking, XML processing, and other common tasks.

WPF has gained popularity for building desktop applications due to its flexibility, rich visual capabilities, and ease of development. It is widely used for building line-of-business applications, media applications, and other types of desktop software.