Cabinet Impulses Response are special files obtained by sampling the response of a cabinet. They are supposed to be much closer to the real thing than programmed emulations. But it all depend of how they have been made and how much power is dedicated to their use. They give back a lot of parameters of the sound of a cabinet, but are still linear, so they won't mimic all cab livelyness, like distortion for example.
You can read a very interesting interview of Mike Grabinski from Red Wirez Impulse Responses about it
here (click).