Flutter is a free and open-source mobile UI framework created by Google that allows building mobile, desktop, and web apps from a single codebase. It uses Dart as its programming language and features hot reload for quickly updating code and hot restart for resetting state. Everything in Flutter is represented as a widget, including both stateless widgets whose state cannot change and stateful widgets that can change in response to events or data. Flutter aims to provide pixel-perfect experiences that match Android and iOS designs.