Qt is a cross-platform application framework for building graphical user interfaces. It provides tools to develop applications that work on multiple desktop and mobile operating systems. Qt uses C++ and offers APIs for developing applications with classic or QML-based user interfaces. It includes modules for GUI, widgets, networking, multimedia, and more. Popular Qt tools include Qt Creator for development, Qt Designer for layouts, and Qt Linguist for translations. Qt sees wide use for applications across domains like embedded systems, mobile apps, and desktop programs.