The NOOKclassic touchscreen is capacitive, so it works on the principle that we have a weak but existent conductive field. So when you touch the touchscreen, it disturbs the electric field that spans the screen, and thereby detects it.
More info here:
http://en.wikipedia.org/wiki/Touchscreen#Capacitive
It's the same as the iPod Touch, iPhone and iPad, but different to a Nintendo DS if that makes it any clearer.