XAML, or Extensible Application Markup Language, is a declarative XML-based language used for creating user interfaces in .NET applications. It was originally introduced by Microsoft as part of the Windows Presentation Foundation (WPF) framework but has since been adopted by other technologies such as Xamarin and UWP (Universal Windows Platform).

One of the key features of XAML is its ability to separate the presentation layer from the application logic, following the Model-View-ViewModel (MVVM) design pattern. This separation allows developers to focus on designing the user interface (UI) using XAML, while the application logic is written in a separate code file, typically in C# or another .NET language. This separation of concerns makes it easier to maintain and test the application code.

XAML is known for its flexibility and powerful features, such as data binding, which allows developers to bind UI elements to data sources, making it easy to display and update data in the UI. XAML also supports styles and templates, which allow developers to define the appearance of UI elements and reuse them throughout the application.

Another key feature of XAML is its support for animations and transitions, which can be used to create rich and interactive user interfaces. XAML supports a wide range of animation types, such as fades, slides, and rotations, making it easy to create visually appealing UIs.

XAML is also highly extensible, allowing developers to create custom controls and behaviors that can be easily integrated into their applications. This extensibility, combined with its powerful features and separation of concerns, makes XAML a popular choice for developing modern, high-quality user interfaces in .NET applications.