Screen fonts are "bit mapped" representations of the fonts so they can be displayed on the monitor (screen) at 72 dots per inch (or pixels per inch). All screen fonts are contained within the Macintosh System and are installed into the System with the font D/A Mover.
On the other hand, Outline fonts are mathematical descriptions of fonts in a language called PostScript. These font descriptions are designed to allow a LaserWriter printer to recreate the font at the highest possible resolution. 35 outline fonts are contained within the ROM's of a LaserWriter Plus, and the LaserWriter II NT & NTX. Other outline fonts are available from software publishers such as Adobe Systems, Inc.
If a document contains fonts that have no corresponding outline fonts, Font Smoothing, in Page Setup, could be used to make the font more presentable and less "jaggy".