Noise
Noise is a problem in digital cameras similar to grain in film cameras: it appears as random coloured dots sprinkled around the image, or as uneven colouration of what should be smoothly-coloured areas.
Noise in a digital camera has several sources, including readout-circuit noise and pixel non-uniformity. However the dominant component is simply the random arrival of photons at the pixel grid of a sensor. This has the technical name shot noise (as in buckshot). Thus, noise is already present in the light striking the sensor, but stands out more clearly when dividing the incoming photons into smaller and smaller "bins." For this reason, sensors with small pixels display the most noise.
Noise reduction software is often employed by digital cameras to reduce the appearance of noise in an image. As CPUs have become faster, more sophisticated noise reduction is able to be viably employed, allowing camera firmwares' software to correct for the technical deficiencies of increasingly noisy photographic sensors.
