ISO

ISO - the sensitivity of the film or sensor of of your camera to the

         light levels falling onto it from the lens.

Low ISO - iso of 100 in bright sunny scenes or scenes where you

                 want to decrease you shutter speed in order to blur

                 motion.

High ISO - iso of 400 to 1600 in low light level scenes where you

                 want to increase the shutter speed of a handheld shot

                 to avoid camera shake i.e. churches, museums, late 

                 evening shots when you do not have a tripod at hand

                 or if you are just trying to introduce some noise into

                 your photograph for effect (black and white portrait).

Note -  As you increase the iso from 100 to 400 you are reducing

            your camera sensor's dynamic range, the sensor's ability

            to capture the brightest and darkest parts of a high 

            contrast scene such as a sunrise or sunset scene. as 

            opposed to an overcast day where these differences are

            much less, a narrow dynamic range.

Note - As you increase your iso you are reducing your camera’s

           Sensor ability to record all the colours present in a scene.

           A setting of iso 100 will record more color range then an 

           ISO setting of 800.