Noise is the digital equivalent of grain in film. It’s random electrical signals captured by the photosites on the camera sensor, and usually this background noise level is so low compared to the brightness of the captured picture itself that you just don’t notice it.
Luminance (contrast) noise
The chief component in image noise and the one that’s most difficult to remove because software can’t easily distinguish between random image noise and real image detail. The result is that the more noise reduction you apply, the more you tend to lose fine image detail, resulting in images with obvious and objectionable ‘smoothing’.