The ISO number is one of the three variables to obtaining the correct exposure, the others being aperture and shutter speed. So when adjusting the ISO you must remember to alter either the aperture or the shutter speed or both.
What is the effect of a low ISO such as 100? Well this means the sensor is less sensitive to light, meaning you need more of it. Also it will give richer colours. An ISO of 3200 will be much more sensitive to light (5x) and will produce less rich colours. When we had film this difference was referred to as graining, the graining increased in size when increasing the ISO number. With digital cameras it is referred to as noise but has a similar if less noticeable effect. In simple terms you will get better quality with a lower ISO.
For most of us you would not notice much of a difference unless you zoom in on the image some considerable distance. The images below show the difference in quality shooting with different ISO settings.
This image was shot with the settings: F2.8 1/3sec ISO 100. You will notice that the colour is stronger in this image.
Just for scale the full sized image would normally be 168 x 112cms or 66 x 44 inches and is shown below.
So after all of that what is the main uses in digital photography. Well for me it is to increase or reduce the sensitivity of the sensor to control the light. In general terms though the advice is that you should not shoot above 400 to maintain quality of the image. I now shoot at ISO 200 and I used to shoot at ISO 400.
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