Basically ISO amplifies the light that goes into the camera. Give me a few days and I intend to do a guide on it to go with the other ones I've done. 100/200 is used for normal well lit situations but values like 800, 1200, 3600, 6400 etc are used in low light situations. The bigger the number the more the light is amplified. The only problem is higher ISO causes more noise to be present in the image. Google 'camera noise' if you dont know what it is