Impulses are audio *captures* of a particular amp, room and mic setup. My understanding is that they can be made by just about anyone with the right software. Think of them as a *snapshot* of an amp (head/combo/cabinet) and then a mic (in whatever position i.e. on-axis, off-axis, near, far, etc) and then a room (probably to a lesser degree). I think what they sometimes do to capture the impulse is record a sine wave of pink or white noise through an amp going from 20Hz to 20KHz. Voxengo Boogex is a nice free impulse program. You can download these impulses from a lot of places on the net, then load them in Boogex and run things like Guitar Rig and others into the Boogex to try out different impulses.
I recommend you check out:
www.guitarampmodeling.com
For more info, they have TONS of free presets for all the major guitar amp emulators out there and lots of impulse files you can download (also free). As well as tutorials and articles to help get you started.
Good luck!
orb.