This systems maps the tonal variances within a single image - originally for shooting development, printing to obtain the correct exposure.
The zone system is the variance of light to dark (White to black) in the form of a 11 shades. Each shade as a given number. Black is "0" and white is "10". The shades in between are variances of dark to very light covering mainly shades of grey.
The central shade (number 5) is representative of "0" the exposure meter. From the centre going darker equates to -1 stop in the exposure scale and lighter equates to +1 for each step taken.
The Zonal system was developed in the 1940's by Ansel Adams and Fred Archer. Adams created the above scale for the use in black and white with vast variances between light and dark areas on a single image. Though the zone systems is just as important with colour photography. Though today very few people develop their own images and light meters for zones are in built in digital cameras.
Originally manual cameras worked on the balance between film speed (ISO), shutter speed and exposure to obtain the overall balance of an image. A hand held spot meter (still used toady by some professional) gives the exact setting for any part of image, allowing the zone system to be used with a little more accuracy.
Modern Digital SLR have computerised zone reading systems built in and create a pretty accurate analytical image map. However as a photographer we still need to be aware when to compensate a small amount by bracketing or adjustments to exposure.
The images above have been reduced in size and quality to fit on the site.