Brightness: Brightness of an image is defined by the higher grey-scale values. Such as, if an image has grey-scale values like 175,200,255 etc, it can be said that brightness of the image is high. That is, the higher the grey-scale value the higher the brightness of an image. Thus brightness of an image is the higher values of the grey-scale levels.
Contrast: Contrast control determines the luminance (proportional to intensity) produces for white, with intermediate values toward black being scaled appropriately. Thus when changing contrast one is actually making a lower value to a lower one and higher value to a much higher one.