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PostScript Printer Description (PPD) files are created by vendors to describe the entire set of features and capabilities available for their PostScript printers. A PPD also contains the PostScript code (commands) used to invoke features for the print job. As such, PPDs function as drivers for all PostScript printers, by providing a unified interface for the printer's capabilities and features. CUPS uses PPD drivers for all of its PostScript printers, and has even extended the concept to allow for PostScript printing to non-PostScript printing devices, by directing output through a CUPS filter.
https://en.wikipedia.org/wiki/PostScript_Printer_Description