A GUI application is an application that uses a GUI ('Graphical User Interface'). In other words: it is an application that uses windows/dialogs with visual elements on which the user can click or navigate to with the keyboard. Most games are GUI applications, with the exception for text-adventures.
Note for Qt Creator users
In Qt Creator, a GUI application is a Qt Creator project type with the following line:
Note that this does not indicate that the program uses a GUI: it might just use non-visual Qt classes, making the program appear as a console application.
#------------------------------------------------- # # Project created by QtCreator 2011-01-14T13:37:45 # #------------------------------------------------- QT += core gui TARGET = CppGuiApplication TEMPLATE = app SOURCES += main.cpp\ mainwindow.cpp HEADERS += mainwindow.h FORMS += mainwindow.ui