An impulse is a "click" or "pop" sound with full amplitude and an infinitely short length. An impulse response is the output of a device (speaker cabinet, effect, etc.) when an impulse is played into the input. The impulse response gives the characteristic time and frequency response of the device it came from. That response can be used by a convolution processor to recreate the output you would get from a device based purely on its impulse response.
The following sites offer cabinet impulse responses:
- some "strange ones" along with some spaces
- 650 assorted cabinet impulses
- Guitar Amp Modeling forum: Impulses