What Is… ISO

ISO sensitivity expresses the speed of photographic negative materials (formerly expressed as ASA).
Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given.
What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the
more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.
And, where you would have needed to physically change to a different roll of film if you wanted a different
ISO speed, digital technology allows you to simply dial one in. In this way, you can record images taken at
different ISO speeds on the same memory card.

more at http://www.photoxels.com/tutorial_iso_print.html

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