PostScript is a page description language, a protocol that is used to communicate between applications like Adobe InDesign, QuarkXPress or MS Word and output devices like laser printers or CtP systems, that are equipped with a PostScript interpreter. PostScript can also be used to exchange data between applications themselves and it forms the basis of the PDF file format.
These pages cover the essentials of the PostScript language, including a fairly elaborate description of its history.
A description of what a page is supposed to look like is irrelevant if you can’t actually turn it into something physical, such as a printed sheet of paper or a printing plate. This article explains how PostScript is interpreted and output. The diagram below gives a basic overview: PostScript data are generated from a layout application when the operator wants to print a job. The data can be sent directly to device with a built-in interpreter, such as a inkjet printer, or the data can be sent to a dedicated RIP that interprets the data for one or more devices such as a platesetter or an imposition proofer.
Links to other sites, newsgroups about PostScript, mailing list and books worth reading.