What is PostScript?
PostScript is a programming language optimized for printing graphics and text (whether on paper, film, or CRT is immaterial). In the jargon of the day, it is a page description language. It was introduced by Adobe in 1985 and first appeared in the Apple LaserWriter. The main purpose of Post Script was to provide a convenient language in which to describe images in a device independent manner. This device independence means that the image is described without reference to any specific device features (e.g. printer resolution) so that the same description could be used on any Post Script printer (say, a LaserWriter or a Linotron) without modification. In practice, some PostScript files do make assumptions about the target device (such as its resolution or the number of paper trays it has), but this is bad practice and limits portability.
Taken from the First Guide to PostScript (alt. location).