Описание тега qdialog
A QDialog is a class from the Qt Toolkit which is the base class for dialog windows.
QDialog serves as a base class in constructing dialog windows. It is different from an ordinary window or widget, because it has a return value, and default buttons. Dialogs can also be made modal or non-modal.
Qt offers a set of default dialogs derived from this class, and allows developers to make custom dialogs by deriving from QDialog. QDialog 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)
The official Qt documentation can be found here for Qt 4.8 and here for Qt 5.