The cmtt font has “curly” quotes, which are pleasing on the eye, but don’t really tally with what one sees on a modern xterm.
The appearance of these quotes is critical in program listings,
particularly in those of Unix-like shell scripts. The
upquote package modifies the behaviour of the
verbatim environment so that the output is a clearer
representation of what the user must type.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=upquot