Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or “fast” film is used which is more “grainy”.
Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the “normal” setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or “noise”. Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.

ISO

The above unmagnified crops of prosumer digital camera images show high levels of color noise at higher sensitivities. Noise is usually most visible in the red and blue channels.

source from: Vincent Bockaert (author of The 123 of digital imaging Interactive Learning Suite)