Grayscale images store values for levels of brightness as opposed to color information. A typical grayscale image is 256 shades of gray ranging from 0 (black) to 255 (white).
Black & White Line Art is typically 2-color (usually black and white) clip art, pen and ink drawings, or pencil sketches. Converting a photograph to line art (as seen in the illustration) may be done for special effects but with only black or white pixels, the details of photographs are lost.
When converting a color photo to B&W, a grayscale image is the goal.
