ISO is one of the three most important variables in proper exposure. ISO refers to the light sensitivity of the sensor in a digital camera. In a digital camera, you can adjust the camera's ISO setting to make the sensor more or less sensitive to light, depending on the situation. The speed or light-sensitivity of a digital camera's sensor is rated in ISO numbers, such as 100, 200, 400, 800, etc. ISO settings can range from as low as 12 to as high as 12500. The ISO is also known as the third exposure control, because increasing the ISO allows you to use faster shutter speeds or smaller apertures.
A lower ISO number will make your camera's sensor less sensitive to light, but will produce a quality image with more detail. Higher ISO settings are generally used in situations where the light levels are low to make the sensor more sensitive to light; however, the cost is a lower quality image containing more digital noise. Low ISO settings require more exposure to light to produce the same image density as a more sensitive ISO. Higher ISO settings require less exposure to record an image, allowing the use of faster shutter speeds and the capability to shoot in low light.
Examine the images to see how they become progressively brighter as the ISO setting increases.