Random ‘speckling’ in an image caused by variations in the light levels captured by the photosites on the sensor. Noise is worse with the smaller photosites on small sensors and at higher ISO settings generally. You can get ‘chroma’ (coloured) noise and ‘luminance’ noise (general ‘grittiness’) the same colour as the background.
Many cameras offer extended (expanded) ISO settings beyond the standard range. These can help you out in an emergency but they’re not designed for everyday use because the image quality is significantly reduced. Some cameras also offer expanded low settings such as ISO 50.
This setting increases the camera sensor’s sensitivity to light. Each ISO step doubles the sensitivity, so it’s easy to use ISO as another exposure control alongside shutter speed and lens aperture. The more you increase the ISO, though, the more the image quality degrades.
One of the two types of digital image noise and caused by random variations in the colour of neighbouring pixels. Colour noise is relatively easy for software to remove without any significant impact on the image quality. Luminance (contrast) noise is the other type, and much more difficult to remove effectively.
On simpler cameras the Auto ISO option simply increases the ISO setting in poor light to keep shutter speeds high enough to avoid camera shake. On more advanced cameras you can program in both the maximum ISO you want to use and the minimum shutter speed, which makes Auto ISO much more useful.