The truth is that Mac use both fonts, PostScript (Type 1) Y Truetype, both for representation on screen and in the printing process, and the main difference between both formats is that the fTrueType fonts are designed in bitmaps (as in an image retouching program), whereas PostScript fonts describe the shape (contour) and size of typeface mathematically, similar to how a vector drawing program works.
The TrueType fonts, or bitmap files, require one file for each font size and style so that it can be printed with quality. To maintain order, all files of the same TrueType font are stored in a special folder whose icon represents a suitcase. The PostScript fonts they use different files for variants of the same font (thick, thin, condensed, etc.), but not to achieve different sizes.
Although the above are the typographic formats par excellence, there are also the Multiple Master fonts and the most recent OpenType (defined in 1996).
The MultipleMaster fonts They only need two files so that they can be used at any size, style and weight, in its version with a cap and without a cap. This format uses three axes for the description of the source, and all of them are supplied with a tool that allows creating instances (custom versions) of the same typography.
Finally, the OpenType format defined by Adobe and Microsoft, its main quality is that it is a multiplatform format (Mac OS and Windows) that combines the best of TrueType and Type 1 fonts, thus avoiding the need to use specific programs for converting sources to use the sources Windows on Mac OS X and vice versa.
If the outline or PostScript fonts are mathematically described and bitmap TrueTypes, OpenType fonts they use both resources. Another improvement of the format is that it allows to define the complete set of characters of the Unicode standard.