Noise

Noise can be summarized as the visible effects of an electronic error (or interference) in the final image from a digital camera. Noise is a function of how well the sensor (CCD/CMOS) and digital signal processing systems inside the digital camera are prone to and can cope with or remove these errors (or interference).
Visible noise in a digital image is often affected by temperature (high worse, low better) and sensitivity (high worse, low better). Some cameras exhibit almost no noise and some a lot and all the time. It’s certainly been the challenge of digital camera developers to reduce noise and produce a “cleaner” image, and indeed some recent digital cameras are improving this situation greatly, allowing for higher and higher ISO’s to be used without too much noise.
Noise can also affect certain color channels more than others, this is because a typical digital camera sensor (CCD/CMOS) is more sensitive to certain primary colors than others (often sensors are less sensitive to blue light) and so to compensate, these channels are amplified more than the others. Noise is also often amplified by the JPEG compression algorithm which reacts badly to a very noisy image (often introducing hue errors which weren’t in the original noisy image). There are several techniques for cleaning a noisy image, and several products which have been developed to specifically perform this task.
Long Exposure “Stuck Pixels“ – Another type of noise often referred to as “stuck pixels” or “hot pixels” occurs only with long exposures (1-2 seconds or more) and appear as fixed colored dots (slightly larger than a single pixel). These stuck pixels can be fairly successfully removed by taking a “dark frame” either before or after the main shot and subtracting this from the original shot, this technique is detailed in Noise Reduction.