Computers with a graphical user interface in the '90s, including Mac OS, used a grid of pixels to draw everything on the screen. Every window, button, text block, etc., would all be drawn one pixel at a time in a one-to-one mapping.
NeXTSTEP instead used Display PostScript, a variation of Adobe's page-description language for laser printers, to draw on-screen windows and text. This meant that text and vector-based elements would always be drawn at the sharpest resolution possible, independent of whatever graphics card or monitor was attached. Actually drawing fonts and other elements was abstracted away from the graphics hardware until the final rendering on-screen.
OS X and iOS use a different technology, called Quartz, but it's based on similar concepts. Quartz has the PDF page description language at its core instead of PostScript. In addition to generating sharp windows and text, the system also gives OS X users the native ability to view and create PDF files.