In digital photography terminology, noise is the digital equivalent of film grain. It shows up on digital photographs as small coloured blotches, usually in the darker areas of an image. Noise often goes overlooked in snapshots, but becomes very obvious if enlargements are made.
Noise is worse in digital photos taken in low light. It can be removed to some extent by software, but a better quality digital camera will usually produce less noise in the first place.
Whereas film grain can add atmosphere to a photograph, digital noise is generally considered to be unattractive. Digital photographers looking for a “grainy” effect aim to start with a clean image (i.e. free from digital noise), and then add the grain effect afterwards using software.
Glossary of Digital Photography Terminology – http://www.digital-photography-tips.net