Flutter is a mobile app SDK developed by Google that allows creating native mobile apps for Android, iOS, and Fuchsia using a single codebase. It uses the Dart programming language and is based on widgets that are similar to but more powerful than views. Apps are built using stateless and stateful widgets where stateful widgets can update their state through setState calls to trigger rebuilds. Flutter uses a widget-based navigation system called Navigator for navigating between screens rather than using activities and fragments like on Android. It supports platform-specific code through platform channels to access native features and asynchronous programming through async/await.