Zone System - A Photography term, used to describe a method of exposing film in such a way as to get the desired expsure that catches all details that the photographer wants.
Quick description: An image (or view) is broken up into lighting zones, which span from black to white. The photographer will set the exposure to ensure that both dark gray and near-white are both captured on the film. If the exposure is not set properly, (under) then dark-gray would show up as black, or (over) then near-white would show up as white. All detail in these areas will be lost, and can not be reproduced on the final print.
Overexposing or underexposing can be used to increase or decrease the number of 'zones' available, and essentially decreases or increases the contrast of the image.
Printing can be adjusted to get the desired effect from properly exposed film by varying exposure time and paper type (high-contrast or low-contrast).
Ansel Adams is one of the best examples of a photographer who used the zone-system to set his exposures. He used his creativity and imagination to create an image in his mind. He would then use the 'zone system' method to achieve that image on film and print. He as a Genius in this respect!