Notes for a presentation given to the Central Maryland Photographer's Guild: An Introduction to Color Space and Color Calibration
Dark frame subtraction is a noise reduction technique for digital photography. Here's the premise:
For short exposure (less than a couple seconds, let's say), the noise of a digital camera's sensor is acceptable. But as exposure times lengthen beyond that, the noise gets amplified along with the subject of the image.
So if you take an exposure that's 15 seconds long, you'll record 15 seconds'-worth of noise along with the subject matter you're photographing.