ISO or Exposure?

For P&S cameras, what is the main difference between changing ISO and changing exposure? I notice that both seem to serve to enhance the brightness of a otherwise too dark image and both need a slower shutter speed. So when should I use which?

Changing the ISO value changes the exposure . Increasing the ISO helps you to shoot in dimmer light.

This intro to expsure tute, of mine, may help even though I was using a DSLR the principals are the same for all cameras.

http://www.poseruniver​se.net …c_Exposure/Expo​sure_1.htm

Ideally, when you change the ISO setting on the camera (in any of the Auto modes), it'll meter just the same way at the new ISO setting. The new ISO setting makes the sensor more (or less) sensitive to the existing amount of light, so the exposure will need less (or more) light to get a "correct" exposure (the more sensitive, or higher, the ISO, the less light it needs to "see" the image). You'll see the shutter speed and aperture change (if your camera gives you this information), but the overall brightness of the picture won't change.

If you use the Exposure Compensation, you actually change the brightness level of the picture, because you're allowing more (or less) light in than the camera would normally choose to on its own, without changing how sensitive the sensor is to the light. You'll see the shutter and/or aperture change here, too, but you'll also see a change in brightness of the image.

If you manually set exposure, you're doing what the camera's meter and the Exposure Compensation settings would, and controlling how much light gets to the sensor. You can get this by varying any or all of the three controls, shutter speed, aperture value, and ISO.