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Hi Andrew,
Flats and Darks are used to calibrate digital images.
Dark frames are taken with the camera in darkness (hence the name) at the same temperature and exposure time as the images they will be calibrating. The dark frame is then subtracted from the images to remove any thermal noise that might have built up during the exposures.
Flat fields are images taken with the optical system pointing at an evenly illuminated light source and at the same focus and camera/lens orientation as the images they will be calibrating. The flats are then divided into the images to flatten the field. This removes vignetting and dust shadows from the images.
I'm not sure to what extent, but I think most consumer digital cameras have internal libraries of calibration frames that are applied automatically when an image is taken, but with astronomical digital cameras the calibration frames must be taken and applied by the user. Both steps are essential in my view, though many of the later chips have very low thermal noise characteristics and some manufacturers claim that darks are not needed with these.
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