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.
The PPD can describe allowable paper sizes, memory configurations, the minimum font set for the printer, and even specify a tree-based user interface for printer-specific configuration.
[1] 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.
CUPS clients usually read the current PPD file from the server every time a new print job is created.