Flutter is an open source framework developed by Google that allows building natively compiled, multiplatform apps from a single codebase. It is approachable for programmers familiar with object-oriented concepts. Flutter is not just a framework or library but a complete SDK that enables creating beautiful, natively compiled mobile, web, and desktop apps. Some pros of Flutter include enabling quick changes, fast development across platforms, and ready-designed and custom widgets. However, cons include apps being potentially weighty, lack of browser support, needing to learn Dart, and smaller resource base requiring writing more from scratch.