Noise

Digital noise is a grainy distortion that spoils pictures shot in low light conditions with high ISO settings or when using a long exposure. Noise appears as speckles which can be black,coloured or bright depending on the scene and the exposure. It is usually worst in the shadows but can also been seen as pink,purple and other colour speckles in what should be a blue sky. A large amount of noise in an image reduces the overall sharpness and clarity of the picture.

Selecting a higher ISO amplifies the output signal from the sensor. The amplification process has difficulty with finding the difference between image signal and noise therefore the noise is also amplified. This helps to explain why high ISO images are always more noisy than ones taken with a lower setting.
In digital cameras this noise can also be affected by a variety of factors for example imperfections in the electronic components of which the camera is made, also the electronic components can  be affected by background electrical fields and electromagnetic radiation. The size of the sensor within a camera and the number of pixels crammed on the sensor will have a huge impact on the amount of noise generated.
Occasionally temperature can affect the amount of noise produced.

The major cause of noise is when there is simply not enough light hitting the sensor causing a sampling error.

Useful information was found the following websites :o)

http://www.photoxels.com/tutorial_noise.html

To try and reduce noise, if at all possible, avoid using long exposures and high ISO settings. There are several noise reduction software applications on the market and some image editing software has a noise reduction filter built in. Adobe Camera Raw offers noise reduction during RAW conversion. (The Noise Reduction section of the Detail tab has controls for reducing noise. Image noise includes luminance  noise, which makes an image look grainy, and chroma (colour) noise, which is usually visible as coloured artifacts in the image.)The Luminance control reduces grayscale noise, and the Colour control reduces chroma noise. Moving a slider to zero turns off noise reduction.

Most DSLR's have a noise reduction feature which can be turned on when required. On the Canon 400D this function comes under the heading of "Custom Functions" with the Long Exp. noise reduction known as C.Fn-2. It is accessed through the Menu button and selecting the Tools 2 tab.