This is a tutorial to learn KDE programming by examples. It assumes you are working with KDE 4, not KDE 3.
KDE is tied together with Qt. KDE classes mostly inherit Qt classes and specify them. KDE 4 is based on QT 4, KDE 3 is based on Qt. When this tutorial introduces Qt classes, this helps you learn KDE programming.
Whenever you have a KDE class and want the KDE documentation for it, point your konqueror to kde:KApplication (in case you want help for the class KApplication). For the Qt class QApplication, you can point your konqueror to qt:QApplication.