Everyday Elements. This site is run by Amanda, a homeschooling mom of four children. Where does she find the time to create such amazing tutorials and Element Templates and Actions?
She recently wrote about ISO. ISO stands for "International Organization for Standardization" and is taken from the Greek word "isos", meaning equal. ISO has replaced the older ASA term.
ISO is an indicator of the film's sensitivity to light. Nowadays, more cameras are digital and fewer use actual film. ISO still refers to the sensitivity to light coming into the camera. To get an excellent overview of what ISO is, how it affects "noise" (the graininess in photos), please link to Amanda's tutorial here on EVERYDAY ELEMENTS.