Xamarin.Forms uses C# to build cross-platform mobile apps (iOS, Android, Windows) from a single codebase.  Crucially, it employs XAML, an XML-based markup language, for UI definition.  XAML allows developers to separate UI design (declaratively defining elements, properties, and layout) from the C# code handling application logic (event handlers, etc.). This separation improves code maintainability and allows for collaborative design.  Xamarin.Forms provides cross-platform controls, customizable via XAML properties (colors, fonts, etc.), ensuring consistent UI across different platforms.  Furthermore, XAML supports data binding for dynamic data display and styles/templates for consistent app appearance, minimizing code duplication.  In essence, XAML streamlines UI development in Xamarin.Forms, resulting in efficient, maintainable, and visually appealing apps.