Online Tutorials: ISO
ISO is a number used to express the light sensitivity of the camera.
On a film camera, the ISO is set by the film and if you want to adjust the sensitivity, you have to change the type of film you use.
In digital photography, however, ISO settings can be adjusted much more easily.
Higher ISO settings allow you to shoot photographs in low light conditions, but this is not as ideal, as high ISO photographs are more susceptible to noise and grainy images than photos shot at low ISO settings.
Doubling the ISO number, for example from ISO 200 to ISO 400, will increase the light sensitivity by 1-stop. This means that if you were composing a photograph with an aperture of f8 at 1/30s, increasing the ISO will mean that either you can have f11 at 1/30s (reducing the aperture size by 1-stop) or f8 at 1/60s (reducing the exposure time by 1-stop) to achieve the same exposure.
Ideally therefore, you should set your ISO setting as low as possible to achieve the correct exposure for the scene you are shooting.
As technology moves on, digital sensors are getting more and more sensitive, so ISO settings are able to be increased to further improve the cameras sensitivity at low light.