on the web:
digital sensors produce artefacts called digital noise
each camera type has different noise characteristics at different ISO's, operating temperature and exposure
in normal photography (meaning bright light photography), noise is usually a secondary consideration, since the signal component easily overwhelms it.
astro-photography differs from normal photography by being signal limited - in almost all cases the bottleneck is the available light input. Usually, the signal is of only slightly greater amplitude than the noise (thermal charge carriers, Johnson noise, schot noise, readout noise, bias noise for any EEs that may be reading this). This is the dominating differential of astrophotography from normal photography.
image noise is caused by several factors, producing different contributions to the noise:
cooling the sensor
minimising sensor over-heating by limiting use of Live Preview, etc.
automatic dark frame subtraction to reduce thermal noise in long exposures
choice of ISO setting - the lower the ISO, the lower the image noise
in-camera noise processing
some Nikon dSLR's apply noise reduction even to RAW files hence the need for "mode 3" to disable this for astrophotography.
most other manufacturers only apply NR to jpegs and not to the RAW files.
most cameras give you an option of turning it on or off and in some cases how aggressive it should be (eg. Olympus E510)
manual dark frame subtraction to reduce thermal noise in long exposures
manual bias frames
noise reduction software techniques:
denoising involves a compromise between reducing image noise and retaining detail and texture.
use of Gaussian blur to minimise appearance of residual noise
wavelet-based noise reduction:
fields of experts
multiple image deblurring and denoising:
utilises the sharp image structures in the noisy image to estimate a kernel of the blurred image, then applies residual deconvolution and gain-controlled deconvolution