What is noise, and why is it used?

What will be an ideal response?

Noise is a term used to describe a blanket of high-contrast pixels that produce a grainy effect over an image.  Adding noise to an image is a technique that designers often use to "finish" a design. A layer of noise over an image can add a heightened sense of sharpness and focus to an image. Noise is especially effective with multiple images used in a montage. Often, in a montage of images from different sources, the challenge is to make the final montage appear to be one piece of artwork, not a composition of different pictures. A layer of noise adds a consistent texture across the artwork and can be very effective in achieving this goal.