An IR (Impulse Response) is a measurement of a systems response to a brief signal. When creating IR out of reverbs or spaces the signal used is either a small click or a sine wave sweep. The small click is great for devices, and the sine wave sweep is suitable for real spaces like a room. When dealing with reverb IRs the measurement is usually a standard wave file in stereo or mono but there are other formats and you can also use two stereo wave files to sample a space to get a reverb impulse response that gives a bit of a different end result. For most spaces a stereo wave file is what sounds the best.
Suitable stuff to make IRs out of
Rooms as in real spaces, software or hardware reverbs and speakers work great to make IR’s out of. When it comes to speakers it wont really sample the distortion if there is any but the frequency and delay of frequencies. For example when recording an IR out of a combo amp you will get the delay between the speakers in the amp and it’s frequency response but not it’s distortion. It still gives great results.
To use a Impulse Response a convolution device is used often called a convolution reverb. It is based on the mathematical convolution operation, and use the pre-recorded audio sample to convolve the incoming audio signal. The process of convolution multiplies each sample of the audio to be processed (reverberated) with the samples in the impulse response file.