Understanding Digital Photography

ISO

These images were made using the same shutter speed and aperture opening. Only the ISO was changed for each image.
The ISO setting determines the camera's sensitivity to light. The lower the ISO number, the more light is needed to properly expose the image. The higher the ISO number, the less light is needed. ISO 200 requires much more light than ISO 6400.
Photographers use higher ISO settings while photographing "action" in low light situations. A higher ISO setting allows the photographer to use a faster shutter speed to stop action for sports photography in a dimly-lit gymnasium, or offers increased depth of field by using a smaller aperture opening. Photographers often use a higher ISO setting to avoid using flash.
Why not always use a high ISO setting? There's a loss of image quality and an increase in "noise" (digital grain) as the ISO is increased. Also, in brightly lit environments there may be too much light and the resulting photos will be over-exposed.