RICHARD BUTLER & RISHI SANYAL, DPREVIEW:
"In the first part of this article, we discussed the fact that a lot of the noise in a lot of your images doesn't come from your camera at all: it is you being able to see the randomness of the light that you've captured and it is almost solely dependent on how much light you were able to capture. The second source of noise is more likely to resemble what most people think of, when they think of noise: it's the background electronic noise that's added to your images by the specific camera you're using. We say this is the source of noise that most people think of because it's much easier to think of noise the way you think of background 'hum' from a Hi-Fi amplifier."
This, along with part one of the article, is a little geeky, but really worth learning. Learn more, here.