WPF is Microsoft's technology for building desktop applications and uses XAML as its UI definition language. It provides capabilities like 2D/3D graphics, animations, vector graphics and a rich typography API. There are different types of WPF applications like traditional desktop apps, navigation-based apps, and Silverlight apps. The core WPF assemblies are PresentationCore, PresentationFramework and WindowsBase and it defines namespaces like System.Windows for the base UI classes and System.Windows.Controls for the built-in controls. XAML allows separating the UI definition from code and is a key part of WPF applications.