PostScript is a page description language optimized for printing graphics and text developed by Adobe. The programming language is a dynamically typed and concatenative. Portable Document Format(PDF) is largely based on PostScript.
A PostScript program can be sent to an interpreter which in the case of a printer will print the document.
PostScript Printer Description file
To describe a printers features and capabilities a vendor creates a PostScript Printer Description(PPD) file. A ppd file works as a driver for PostScript printers as it also can contain PostScript code for invoking feature of the printer.
CUPS and PPD
Common Unix Printing System(CUPS) uses PPD drivers for all PostScript and non-PostScript printers. A CUPS-ppd file is not a standard ppd file because it adds attributes and extensions. For example
cupsFilter defines a conversion rule which takes a mime type as input and a conversion program to run. The program in this case can be a vendor specific driver binary.