Flutter is an open-source UI software development kit (SDK) created by Google. It is used to develop applications for Android, iOS, Linux, Mac, Windows, Google Fuchsia, and the web from a single codebase. Flutter was first unveiled in 2015 as a project called “Sky” and officially launched as Flutter in May 2017.

One of the key features of Flutter is its use of the Dart programming language. Dart is a modern, object-oriented language that is easy to learn and offers features such as a just-in-time (JIT) compiler for fast development cycles and an ahead-of-time (AOT) compiler for fast execution of production code. Flutter’s use of Dart allows developers to write code that is expressive, efficient, and easy to maintain.

Flutter also offers a rich set of pre-built widgets that make it easy to create beautiful and highly customized user interfaces. These widgets follow the Material Design guidelines for Android and the Cupertino design guidelines for iOS, ensuring that apps have a native look and feel on each platform. Developers can also create their own custom widgets to further customize the user interface of their apps.

Another key feature of Flutter is its hot reload functionality. This allows developers to make changes to their code and see the results instantly, without having to restart the app. This can greatly speed up the development process and make it easier to experiment with different designs and features.

Flutter also has strong support for asynchronous programming, allowing developers to write code that is non-blocking and responsive. This makes it easy to create apps that can handle complex tasks such as network requests and database operations without freezing the user interface.

Overall, Flutter is a powerful and versatile SDK for building cross-platform apps with a native look and feel. Its use of Dart, rich set of widgets, hot reload functionality, and strong support for asynchronous programming make it a popular choice among developers looking to create high-quality mobile and web applications.