In a GUI, text is treated as a graphic, not as a fixed set of characters like in a typewriter (remember those?). So they can differ in design, size, and even character spacing. In other words, they are digital fonts -- malleable in a way that physical typefaces never were.
The original Mac fonts were bitmaps, rendered by an OS service called QuickDraw in a fashion similar to dot matrix printers. But in 1985's Mac OS and LaserWriter printer, Apple adopted Adobe's PostScript technology, which treats fonts like drawings so they can be manipulated in all sorts of visual ways. Aldus released PageMaker, which used PostScript for more than text, and the desktop publishing revolution began.