PostScript is a page description language - a programming language, much like those that software engineers use to build applications. In fact, you can do an experiment to prove this to yourself. Save this file, and open it in a word processor. In the earliest days of PostScript, drawings could be created only by manually typing in the PostScript language. Illustrator allows the designer to draw with graphic tools while it automatically writes a PostScript program in the background.
Every programming language needs a processor to run or execute the code.
In the case of PostScript, this processor is a combination of software and hardware which typically lives in a printer, and we call it a RIP - a Raster Image Processor. A RIP takes in PostScript code and renders it into dots on a page. So a PostScript printer is a device that reads and interprets PostScript programs, producing graphical information that gets imaged to paper, film, or plate. Compared to page layout applications from other vendors, which use a bitmapped preview, this is a much more press-accurate way of doing things. One way to use PostScript is to print a file to disk and save it as one single PostScript file which can be sent on to a print service provider.