What is ISO?
ISO is the level of sensitivity of your camera to available light.
ISO 50-100. Suitable for bright light (like outdoors on a sunny day).
ISO 200. Great for overcast or cloudy days.
ISO 400 and 800. Use these values when the light is getting dim but it is not yet night.
ISO 1600 and above. Use for indoor or night shots. Also useful to freeze the action in sports shots.