In the blender manual the options for camera data nodecamera data node are explained like this:
View Vector
A Camera space vector from the camera to the shading point.
View Z Depth
How far away each pixel is from the camera
View Distance
Distance from the camera to the shading point
So, what is the shading point? How is each of these options different form the other, and in which cases should one be used instead of the other?