In the old days, one of the first decisions you made when buying film was the ISO or ASA rating. One would purchase a film for daylight use at a rating of 100 or lower light conditions at about 400. The rating stood for the film's sensitivity to light. A higher number meant the film was "faster" and you could use it to shoot in lower light conditions.
click thumbnail to enlargeShooting during any kind of performance indoor requires a high ISO setting. Flash may not be practical or allowed in such instances© Ray Villalobos 2009
ISO comes from the International Organization for Standardization, the group that develops this and other standards for various uses. The letters ISO are not an exact acronym and is what they chose as a short form of their name. Like with typical shutter speeds and F-stops, the typical ISO values are used to half or double the amount of light. So an ISO setting of 200 means the sensor is twice as sensitive to light as ISO 100.