ISO...
A camera's ISO function sets the light sensitivity of the camera's image sensor (this is similar to the speed rating of FILM). ISO settings are often rated at 100, 200, or 400 but go as high as 800, 1600, and even 3200 on some advanced models. A lower ISO setting is used when capturing overly bright scenes, since it reduces the light sensitivy of the image sensor. This is ideal when shooting at the beach, on a ski slope, or under the midday sun. A higher ISO settings is often used when shooting under dimmer conditions (cloudy days, indoors, etc.) since it increases the light sensitivity of the image sensor. As brightness in a scene is decreased the camera tries to compensate by slowing the shutter speed which in turn lets in more light but increases the risk of motion blur. To prevent this, you can increase the ISO or sensitivity of the camera, which allows the camera to select a higher shutter speed, thus reducing motion blur.

Because this increases shutter speed, its good for fast action. And you can use this for those rolling shots that you see people taking of cars.