ISO (light sensitivity) used to be determined by the film that was put in your camera, but in digital photography it has become another tool – an adjustment that can be changed for every shot. ISO is one of the most important settings in low light conditions.
High ISO gives you the ability to shoot at faster speeds, but opens up the possibility of getting more noise (or graininess) in your fotos. Some cameras have the ability to set ISO really high, but going over 3200 isn’t recommended. Low ISO corresponds to slower shutter speeds but also provides for very crisp pictures, so use a low ISO of 200 or so when plenty of light is available.