Can someone just give me the general rundown on what the ISO numbers mean, and why you would want to shoot with a high ISO in one situation and a low ISO in another?
My camera has ISO 100, 200, and 400, and auto. If I shoot with ISO 100, do I need to increase the exposure a little, or something?
ISO, in digital, basically reflects the "sensitivity" of your sensor. In film, it means that for a given f stop, a faster shutter speed can be used as the ISO number goes up. It's basically the same idea in digital, but sometimes, depending on your camera, the quality goes slightly downhill as the ISO goes up. It's primarily an indicator of "film speed", and higher ISO means faster film means less required exposure time.
ISO is a measurement of the sensitivity to light. With digital cameras, it's like turning up the gain on a microphone - it boosts the signal, but also boosts the inherent background noise.
you'll get the cleanest signal with the lowest iso. but in situations where your shutter speed is too slow because it's too dark - like indoors or at a concert or dusk when you need to capture something moving, a higher iso will give you a higher shutter speed.
each doubling of iso buys you one f/stop worth of shutter speed, at a given aperture.
By the way, if you exposure is being set automatically, the camera should be correctly exposing based on the ISO setting. If you are totally manually exposing, the exposure indicator should also be indicating based on the ISO setting, so you shouldn't have to make any intentional "adjustments" based on the ISO you are using.
Basically, changing from 100 to 200 halves your shutter time. Changing it to 400 halves it again. This is done by digital boosting of the image information (x2 or x4, in our case.)
Also see this article, which explains the relation between shutter time, aperture and film speed
