ISO : The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy"

Aperture: Aperture is also known as 'f-stop' and you can think of it as being similar to the pupils in your eyes

Shutter speed: Shutter speed is how long the sensor in your camera is exposed to light

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References:
http://www.dpreview.com/∞
http://www.knowyourdslr.com/dslrbasics/startersguides/12708/dslr_starters_guides_aperture.html∞