Qt is a cross-platform application development framework for desktop, embedded and mobile.
Supported Platforms include Linux, OS X, Windows, VxWorks, QNX, Android, iOS, BlackBerry, Sailfish OS and others.
Qt is not a programming language on its own. It is a framework written in C++.
A preprocessor, the MOC (Meta-Object Compiler), is used to extend the C++ language with features like signals and slots. Before the compilation step, the MOC parses the source files written in Qt-extended C++ and generates standard compliant C++ sources from them.
Thus the framework itself and applications/libraries using it can be compiled by any standard compliant C++ compiler like Clang, GCC, ICC, MinGW and MSVC.
We often hear about Qt to build cross-platforms applications easily.
The user must be aware a license is required if the app is developed solely for an enterprise use.
Qt can be used freely while the code is open-source and distributed under (L)GPL license.