I always get to the question, so what is it? Nasim says, ISO is the level of sensitivity of your camera to available light - whether we're talking film or image sensor. To me, that even relates to how Chris and Eric think of ISO. Chris - increase read voltage on the image sensor and Eric - increase the amplification at higher ISOs. Increase the voltage, amplification, sensitivity and you can see we're in for a tradeoff. Greater light sensitivity at the expense of noise on this still analog device, the image sensor. BTW, Chris does that mean operating at higher ISO levels in low light conditions, we are creating a greater draw on the battery?
There is a really good explanation of what happens, and in what order, with Auto-ISO - it's either in Digital Darrell's or Thom Hogan's D800 book. I haven't dug it out this morning. I like to use Auto-ISO, but others may prefer not.
If we're in a low light situation, Eric's mention of aperture priority makes a lot of sense to me. Use a fast lens, open the aperture and crank up ISO to try and keep the shutter speed up.
If we choose to use Manual, and even manual ISO, aren't we in the realm of seeing what kind of exposure preview we have available or resorting to exposure tables? Eric mentioned Exposure Comp changes the scale, but I don't understand?