Consumers are beginning to see how megapixels don't make a better camera, so camera manufacturers (or marketers) are looking for other numbers to slap on the front of the box. One number that many cameras have begun to tout is the ISO. In some cases, manufacturers claim "low-light" or "boost" modes.
In the days of film, ASA or ISO was a measure of how sensitive the film was to light. If you remember, 100 was meant for shots out in the sun, 200 was for cloudy days, and 400 was for indoors.
In digital cameras, the numbers mean pretty much the same thing, but with one crucial difference – you don’t actually make your sensor more sensitive when changing your ISO. All you are really doing is amplifying the signal from the sensor – and this is bad.