Adobe Postscript Printer Description (PPD) files describe how to use the special features for a specific Postscript printer. They are suppose to be human-readable, but they are really only readable by PostScript gurus. It is the responsibility of the printer manufacturer to supply PPD files. However, the Adobe mail server has many of them. (see the Section 6, ``About Adobe'')
Adobe Technical Document 5003 - PPD Specification describes how to read and parse PPD files. You can find it on the Adobe mail server, or by anonymous ftp to ftp.adobe.com:/pub/adobe/Documents/5003.PPD_Spec.ps.