How to use ISO is a question we hear a lot. ISO is the third ball in the juggling of aperture and shutter to get the correct exposure. Of course the big question is 'Why do I need THREE exposure controls?'
Well aperture is a very creative camera control because it affects depth of field so you need to set it according to what you want to achieve creatively. The shutter controls how much light gets in but if it's abit slow you might end up with a fuzzy picture because of camera shake.
By increasing your ISO the camera becomes more sensitive to light and camera shake becomes less of a problem.
This clip is from our Photography Training DVD available from http://www.photographycourses.biz/photography_dvd.html . ISO is a kind of bonus exposure control which can really get you out of trouble.