Qt Quick is a modern user interface technology that separates the declarative UI design and the imperative programming logic. Instead of the traditional C++ APIs of Qt, the presentation layer of the application is written with a Qt-specific declarative language called QML.
Qt Quick applications have a solid C++ backend, even though the UI is written with QML. This allows direct native device access from the QML layer as well as extends QML with C++ written graphics, like OpenGL objects. The application logic and data access can be written with the cross-platform Qt C++ APIs.
Declaring UI layouts with QML is highly productive and does not require hardcore programming skills making it a common language for both designers and developers. High abstraction level and super-quick workflow enable rapid prototyping of the UI.