The Foomatic database is an amazing piece of ingenuity in itself. Not only does it keep the printer and driver information, but it is organized in a way that it can generate PPD files on the fly from its internal XML-based datasets. While these PPDs are modeled to the Adobe specification of PPDs, the Linuxprinting.org/Foomatic-PPDs do not normally drive PostScript printers. They are used to describe all the bells and whistles you could ring or blow on an Epson Stylus inkjet, or an HP Photosmart, or what-have-you. The main trick is one little additional line, not envisaged by the PPD specification, starting with the *cupsFilter keyword. It tells the CUPS daemon how to proceed with the PostScript print file (old-style Foomatic-PPDs named the cupsomatic filter script, while the new-style PPDs are now call foomatic-rip). This filter script calls Ghostscript on the host system (the recommended variant is ESP Ghostscript) to do the rendering work. foomatic-rip knows which filter or internal device setting it should ask from Ghostscript to convert the PostScript print job into a raster format ready for the target device. This usage of PPDs to describe the options of non-PostScript printers was the invention of the CUPS developers. The rest is easy. GUI tools (like KDE's marvelous kprinter or the GNOME gtklp xpp and the CUPS Web interface) read the PPD as well and use this information to present the available settings to the user as an intuitive menu selection.