Unix applications that print generate PostScript files as output. PostScript is not a simple file format; rather, it is a page description language. Therefore, PostScript documents are programs, so you need an interpreter to view or read them. Most mid-to high-end laser printers understand PostScript, as do certain inkjet printers, but the cheapest printers do not. If you have such a printer (and who doesn't like to buy cheap hardware?), your computer must perform the extra step of rasterization before the document is sent to the printer.
When you rasterize a PostScript file, you convert its vector graphics into a bitmap. On Linux, the Ghostscript program does this work. Ghostscript can produce output for almost any printer on the market, but it also has many other uses because it is a full-featured PostScript interpreter. See Section 12.6 for more information.
If you are interested in PostScript as a programming language, PostScript Language Tutorial and Cookbook [Adobe 1985] is a good introduction to the language, and PostScript Language Reference Manual [Adobe 1999] serves as a comprehensive guide to the language features. However, if you just want an example PostScript file for tests, use a Web browser's print-to-file feature.