PostScript is a page description language (PDL) that describes a page's text and graphical content. It can be used to define the appearance of graphics and text for both screen and print. The language was developed by Adobe in 1984 and has since gone through many revisions and updates.
Before PostScript was introduced, publishing systems relied on proprietary typesetting systems, which often caused incompatibilities between computers and printing systems. In fact, before the days of PostScript, pages that incorporated text, images, and line art had to be manually assembled on a paste-up board and then photographed. The resulting picture was sent to a printing plate, which was used to make copies of the document -- pretty archaic compared to the simple printing options available today.
Adobe PostScript makes it possible to produce high quality page content that can include text, images, and line art in a standard format compatible with multiple devices. For example, PostScript (.PS) files will print the exact same way from different PostScript compatible printers. They can also be opened using Adobe Acrobat and will look the same on Macintosh and Windows platforms. In fact, the evolution of PostScript led to the development of Adobe Acrobat, which creates PDF documents.