key or main light is basically when the subject is facing directly to the camera and to get the best possible picture we look for the triangle of light which is the shadow that is casted by the nose.
Background light is when a light is pointed towards the background and adds depth to the photograph, we don’t look for a halo though.
Broad Light is by placing the light on the side of the subject which creates shadow on the opposite side of the face where the light is not facing.
