When you photograph with film you have to choose between black-and-white, color negative and color slide film. In addition, each film had multiple speeds, or sensitivity ratings, denoted by an ASA number. A popular color slide film like Kodachrome 64 was considered a "slow" film. The 64 stood for its ASA. It was mainly an outdoor film and was considered National Geographic's film of choice. Fujichrome's 800 ASA color slide film, a "fast" film, enabled a photographer to shoot indoors, in less than ideal lighting conditions. For comparison, here are exposures for an indoor scene, using different film speeds at f4.0.
ASA 64 - 1/8th second @ f4.0
ASA 400 - 1/60th
ASA 800 - 1/125th
ASA 1600 - 1/250th
Each time you double a film's sensitivity(ASA), you can reduce the shutter speed by half. You could halve your aperture instead, if you chose. Changing your film's ASA is another exposure control along with shutter speed and aperture.
But with digital cameras, there's no need to change film. Digital cameras record images on electronic image sensors and the sensors sensitivity can be adjusted, like changing to a different speed film. Measured by ISO ratings, sensor sensitivity is similar to film ASA.
The higher you set your ISO, the more sensitive the sensor is to light and therefore, less light is needed to make an exposure. It's a very handy way to extend the exposure range of your camera in low light. Let's say you want to catch your son or daughter in a swim meet dive. You've determined that a proper exposure is 1/125th at f4.0 at ISO 800. Unfortunately, your Aquahawk dives too fast and is blurred at 1/125th so you have to switch to 1/250th. A bump up in the ISO from 800 to 1600 will compensate for halving the shutter speed.
But here's the bad news. Any increase in ISO sensitivity on digital cameras will increase grain or "noise" in your images. Noise is the degradation of image detail and clarity. My recommendation is to avoid using ISOs above 400 on point-and-shoot cameras because of their smaller sensors. If you own a digital single lens reflex, you can probably get acceptable results with 3200 ISO.
Next week I'll write about how the end use of your photographs can determine how much noise is acceptable.