ISO from 100 to as high as 6400 are obtained by dial adjustment

Higher ISO are mechanized by increasing the gain of the measured signals

Noise increases as well as signal, similar to increasing the volume on a radio to get a weak station

Noise corresponds to grain in film as the penalty for higher ISO

The larger size sensors and higher light-gathering power in DSLRs gives them a great advantage over consumer digital cameras for higher ISO

Noise measurements vs. ISO are included in the camera reviews at dpreview.com

Ruddy Duck