The Mac OS renders TrueType fonts for screen display using the same font outlines used for printing, creating shapely screen type. The Mac OS won't perform these services for PostScript fonts. Instead, it displays only screen fonts, bitmapped equivalents of PostScript printer fonts drawn for specific point sizes. For the OS to handle a PostScript font, at least one screen font for it must be in the Fonts folder. If you ask for type sizes that don't correspond to installed screen fonts, the system creates screen type by scaling an available screen font.
Results vary from ugly to hair-raisingly ugly.
If the corresponding PostScript printer font is available, though, printed type looks fine. ATM creates screen type by scaling a font's printer outlines and can also apply grayscale anti-aliasing. Macs also need ATM to use PostScript fonts on a non-PostScript printer, ATM rendering type for the printer just as it does for the screen, only at higher resolutions. The Mac OS renders TrueType fonts for non-PostScript printers. Most Mac applications only inventory fonts when starting up.