With film cameras, remember the decision as to which film to use? Films are rated in ISO numbers, where ISO 100 is for bright outdoors, ISO 400 for indoor and outdoor use, and ISO 800 and 1600 for low-light conditions. Some digital cameras have the ability to adjust the ISO setting in the same way. With digital, ISO is the sensitivity of the CCD to light, the higher the ISO setting the more sensitive to light the CCD is. The lower the ISO the less sensitive to light the CCD is.
The Teeter Totter relationship still stays the same, it is just the range that changes.