ISO is a range of numbers that represents the light sensitivity of your digital camera's sensor. (ISO is an acronym for International Organization for Standardization, which sets those numerical values.) ISO is digital photography's equivalent of the ASA numbers in film-based photography. The sensor is less sensitive to light at the lowest number. For example, 100 ISO is typically a 'normal' setting, which produces a sharp picture because the graininess of the image is fine. Think of sandpaper as a metaphor. A higher number will be more sensitive to light, which makes for a grainer photo, much like coarse-grain sandpaper.