In photography, Black & White photographs are actually shades of gray. In digital imaging these B&W images are called grayscale to differentiate them from black and white line art. 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 may be done for special effects but with only black or white pixels, the details of photographs are lost. http://desktoppub.about.com/od/scanninggraphics/ss/color_to_bw.htm
The opposite of a negative (see below) is a positive (also called "transparency," "slide" or "reversal"), which does not inverse the colors or monochromatic shades of a scene. click here for more information
In photography, a negative is usually photographic film coated with chemicals that, upon photographic exposure, cause the material to record the colors or monochromatic shades of the scene in inverse, negative form. This process occurs inside a camera. The resulting "negative" needs to undergo film developing in order to be made insensitive to light. Then, it can be used to produce a print, in which the colors or shades are reverted to their original form. click here for more information
Color negatives, intended for use in creating positive color enlargements on specially coated paper, are now the most common form of film (non-digital) color photography, owing to the introduction of automated photoprinting equipment.