The QDialog class is the base class of dialog windows.
A dialog window is a top-level window mostly used for short-term tasks and brief communications with the user. QDialogs may be modal or modeless. QDialogs support extensibility and can provide a return value. They can have default buttons. QDialogs can also have a QSizeGrip in their lower-right corner, using setSizeGripEnabled().
Note that QDialog uses the parent widget slightly differently from other classes in Qt. A dialog is always a top-level widget, but if it has a parent, its default location is centered on top of the parent's top-level widget (if it is not top-level itself). It will also share the parent's taskbar entry.
Please note, that QDialog inherits QWidget. It behaves mostly like QWidget but adds new capabilities. You can think of QWidget as an atom class for a user interface. Any Qt object that you see on the screen inherits QWidget.