ISO stands for International Organization for Standardization, a European establishment that sets guidelines on a wide variety of subjects. Traditionally in photography, ISO is the measurement of how sensitive your film is to light. In digital photography, it controls how sensitive your digital camera is to the incoming light.
Most cameras have an automatic ISO option. If yours doesn’t or you prefer to use a manual setting, an ISO setting of 100 is a good all-around number for an evenly exposed image on a sunny day. The darker the background, the higher you should set your ISO. Once you start to get into the higher ISO settings anywhere from 400-1600, you start to loose smooth saturated images and you begin to gain noise in your photographs.
Uses of a low ISO (50-400)
Uses of a high ISO (400-1600)