>>2621167>>2621210The purpose of stacking is to increase your signal to noise ratio (and your dynamic range). The stacking software detects stars on each image and align all your frames. It then keep the real photons from space (stars, nebulae) and add them together to increase detail. It also removes the noise (random electronic noise, skyglow..) by averaging it out. (noise is usually random on each frame, real photons are not)
my ISO settings change for each object, basically for very faint gas clouds like in this pic I have to shoot at 6400. I can lower it too 400-800 for brighter objects (planetary nebulae), M42 orion nebula etc.
For the exposure time before the stars start trailing, it depends on your focal length, accuracy of your polar alignment and quality of your mount. I have a cheap setup and my typical exposure time is between 50 and 90 seconds at 1000mm. With a good polar alignment and autoguiding you can push this number to 3 or even 5 minutes. The magical number for deep sky astrophotography is 2 minutes.
http://keithwiley.com/astroPhotography/imageStacking.shtml 