It enables declaring user interfaces using Extensible Application Markup Language (XAML) technology.

WinUI is one of the multiple UI frameworks provided built-in for the Windows Runtime; the others being HTML5 (e.g., via WinJS) and DirectX.

WinUI 2 is an extension library for UWP XAML containing controls and styling that match the Windows 11 design language.

It is shipped through NuGet and is distinct from the UWP XAML framework, which provides the actual rendering engine; though, they may be treated as synonyms.

WinUI 3 decouples WinRT XAML from the operating system as a separate package to be updated quickly and make new features work on older versions of Windows.