ISO - sounds impressive enough, but what is it?
ISO indicates your digital camera’s sensitivity to light. The higher the number, the less light is needed to take a photo that is well exposed.
In good light, you will not need to increase your ISO.
However in low lighting conditions your camera will need some extra help - there are 2 ways of doing this:
Use a longer shutter speedBUT... always one isn't there?!
This is not always the best option, especially if the subject of your photograph is moving, or you do not have a tripod to keep the camera steady! The resulting image would just be a blur - whoops!
Increasing your ISO increases your cameras sensitivity to light, allowing it to use faster shutters speeds in darker environments, helping to prevent the risk of blurring or camera shake.
The limitations of high ISO is technology, currently high ISO introduces unwanted noise. So most photographers will opt for a longer shutter speed.
Example of noise due to high ISO setting
However the latest 'top end' cameras are beginning to offer some very useful high ISO, so it's worth experimenting with your camera to find its and your comfort range.
Most cameras have an “Auto ISO” setting - so once you find the 'limit' you are happy with you can set it to that. Then your camera will automatically measure the amount of light in the scene and change the ISO accordingly so that the shutter speed doesn't get too slow.