Xamarin.Forms is an open-source framework for building cross-platform mobile applications using a single C# codebase. It allows developers to create native user interfaces for Android, iOS, and UWP (Universal Windows Platform) applications. Xamarin.Forms enables the use of shared code for the app's business logic, while native UI controls are rendered for each platform. It supports the MVVM (Model-View-ViewModel) architecture and uses XAML for defining the UI. Xamarin.Forms simplifies app development by providing common controls, layouts, and a unified API for accessing platform-specific features, offering faster development with less code duplication.