Hi Ian
Darks are exposures with the lens cap on. These should be as long as the exposures you take. This allows you to get a baseline of noisse generated by the camera.
2, Lights are the actual exposures you make to get a picture.
3. Flats are exposures of an evenly illuminated screen , light box or dusk or twilight sky.These enable you to get rid of vignetting and dust donuts etc.
I found that AIP4WIN at USD100 was good value for money as you get a 700 page hard cover text book on astronomical imaging plus the processing software. The text alone IMO is worth the money as it explains all the nitty gritty of calibration and operation of CCD and DSLR cameras.
