The Quad View is a special view with the camera, front, top, and right side views. I especially like it for setting up lighting.
Press the Ctrl key, the Alt key, and the Q key at the same time.
Press the Ctrl key, the Alt key, and the Q key again.
With the 3D View in Quad View, press the 5 key on the NumPad.
Press it again. What change do you see?
When you press the Ctrl key, the Alt key, and the Q key, Blender toggles the Quad view on and off. The Quad view gives you three Orthographic views and the Camera view, while the Ortho views do not change. When you press the 5 key on the NumPad, the camera window does toggle between the Ortho and Persp views.