A driver is a program that takes as input a DVI file (DVI files) and (usually) produces a file that can be sent to a typographic output device, called a printer for short.
A driver will usually be specific to a particular printer, although any PostScript printer ought to be able to print the output from a PostScript driver.
As well as the DVI file, the driver needs font information.
Font information may be held as bitmaps or as outlines, or simply as a
set of pointers into the fonts that the printer itself 'has'.
Each driver will expect the font information in
a particular form. For more information on the forms of fonts,
see pk files,
tfm files,
virtual fonts
and Using PostScript fonts with TeX.