ISO is a measurement of the sensitivity to light of a light sensitive surface, whether film or digital sensor. It is also one leg of the "Exposure Triangle". The other legs are f-stop (aperture) and shutter speed. All three must work in harmony to produce a correct exposure.
A low ISO - 50, 100 - is very insensitive and requires a lot of light. We use it on a sunny day and when we want the best image quality. In film its said to have small, almost invisible, grain; in digital its said to have very little, if any, noise.
A high ISO - 200, 400, 800, 1600 - is more sensitive and requires less light. We use it on an overcast day (200) or for sports/action (400), or for low-light, non-flash photography (800, 1600). In film a higher ISO is said to have more grain; in digital its said to have more noise.
If our goal is to make large prints from our negatives or digital files we will use a low ISO, with our camera on a tripod. If we're using digital then we'd also use our camera's highest resolution.
I suggest these books to help you better understand your camera and how to utilize it to its maximum potential:
"Understanding Exposure" & "Understanding Shutter Speed", both by Bryan Peterson.
Photographer/author David Busch has written a Guide for your camera. Buy it since it will help explain those parts of the Owner's Manual that could have been better written. http://www.dbusch.com
"Digital SLR Handbook" by John Freeman.
"The Art of Digital Photography" by Joseph Meehan.
Its absolutely imperative that you READ & STUDY the Owner's Manual for your camera.
Photography, like any subject, requires study. If you don't study, you fail.
If you have more questions you may email me.
