What is Contrast ?
Contrast is the exact opposite of something. A nice boy is a sharp contrast from a mean boy, and the colour white is a contrasting colour to black. A contrasty photo Would have a lot of dark and light in it but the mid range colours would be consumed by either dark or light. In black and white photography there would be little in the way of grey and black and white would dominate the scene.
The Light-Dark Contrast
The light-dark contrast describes the brightness of a colour; it is also called tonal value. The following illustrations demonstrate the brightness of the colours on the colour wheel. The tone of the circles informs you about the degree of the tonal value; yellow is the lightest and violet is the darkest colour: