The aperture, aka f-stop, which is the size of the opening in the diaphragm inside the lens. The smaller the number, f1.2, f1.4 the larger the opening and the more light admitted. The larger the number, f11, 16 the smaller the opening and the less light admitted.
The shutter controls how long the light admitted by the lens is allowed to expose the light sensitive surface, whether film or digital sensor. The more light admitted the less time the shutter remains open (fast shutter speed). The less light admitted the longer the shutter remains open (slow shutter speed).
ISO is simply the measurement of the sensitivity to light of a light sensitive surface, whether film or digital sensor. The lower the ISO the less sensitive; the higher the more sensitive. For maximum image quality a low ISO is best. For low-light or fast action a higher ISO is better.
Here are some hypothetical examples:
ISO 100
f1.4 @ 1/500
f2 @ 1/250
f2.8 @ 1/125
f4 @ 1/60
f5.6 @ 1/30
ISO 200
f1.4 @ 1/1000
f2 @ 1/500
f2.8 @ 1/250
f4 @ 1/125
f5.6 @ 1/60
ISO 400
f1.4 @ 1/2000
f2 @ 1/1000
f2.8 @ 1/500
f4 @ 1/250
f5.6 @ 1/125
As in all things, there are no solutions, only trade-offs. The lower the ISO the better the image quality but you either shoot at a wide aperture or slow shutter speed. The higher the ISO the lower image quality but you can stop down more or use a fasterer shutter speed.
