A DVI file (that is, a file with the type or extension .dvi) is
TeX's main output file, using TeX in its broadest sense to
include LaTeX, etc. 'DVI' is supposed to be an acronym for
DeVice-Independent, meaning that the file can be
printed on almost any
kind of typographic output device. The DVI file is designed to be
read by a driver (DVI drivers) to produce
further output designed specifically for a particular printer (e.g., a
LaserJet) or to be used as input to a previewer for display on a
computer screen. DVI files use TeX's internal coding; a TeX
input file should produce the same DVI file regardless of which
implementation of TeX is used to produce it.
A DVI file contains all the information that is needed for printing
or previewing except for the actual bitmaps or outlines of fonts, and
possibly material to be introduced by means of
\special commands.
The canonical reference for the structure of a DVI file is the source of dvitype.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dvi