What's the difference between QMainWindow, QWidget and QDialog?

QWidget :is a base class for all other GUI elements in QtWidgets module. It can constitute a window by itself, or be part of a QLayout, or just a member of parent-child hierarchy.

QDialog :is usually used to - surprise! - display a temporary dialog when user input is required.

QMainWindow :is a convenience class that can be used as the main window of your application. It has some nice features built-in: a status bar, tool bars and a menu bar