Impulse responses. If I wanted to confuse you, I'd send you to Wikipedia:
http://en.wikipedia.org/wiki/Impulse_response
Basically it's a way of modelling a system (like a room's reverb, the sound of an amp or preamp) by putting a short burst of full frequency sound through it, recording the result and then subtracting the original sound to leave you with only the difference - the impulse response. this response can them be used to model how another signal (Like a guitar sound) would sound processed by the same device/space. Great for "real" reverb sounds.
http://www.soundonsound.com/sos/apr05/articles/impulse.htm