Noise
Noise is the digital equivalent to graininess in film. Higher ISO settings generally lead to higher noise levels in the final image. With digital cameras, the ISO setting is a gain on the pixels to make them more sensitive to light. A lower light level is increased at higher ISO settings. Photographers often shoot at the lowest possible ISO setting. That setting is determined by the amount of light available and the shutter speed or aperture desired for the final image. If the ISO setting results in noise, the photographer may post process the image to decrease the noise, but generally loses sharpness/details in the image in doing so.
More pixels on a sensor does not always mean a better image due to higher noise levels for a given amount of light reaching each pixel. Many digital camera manufacturers have reached a practical limit to the megapixels on their sensors, and instead of continuing the megapixel wars, they are seeking improvements to noise reductions for the pixels they have. Many newer DSLRs have low noise at high ISO settings.
