Noise in digital photography[]
Light is noisy by nature and noise in digital photography is almost entirely a function of how much light has been captured. The more light is collected, the larger the ratio of signal and noise (SNR) will be - thus less noise will be observed. The image sensor itself adds very little noise to the signal and is irrelevant outside of very small exposures.
Noise most commonly appears as either random coloured dots sprinkled around the image, or uneven colouration of what should be smoothly-coloured areas.
Noise reduction software is often employed by digital cameras to reduce the appearance of noise in an image when taking JPG images, especially is high ISO settings have been used - sometimes also the unprocessed raw-files may have often unwanted noise reduction.