[gcr]

I thought it was postscript internally?

[dmd]

NeXT and NeWS were Display Postscript. OS X / Quartz is PDF.

[Someone]

It is more accurate to say that both Quartz and PDF (and Postscript) use the same primitives (cubic Beziers, color models, graphic state, etc)

PDF the file format adds many, many things to that (forms, encryption, DRM, notes, a JavaScript engine, reflow information, etc)