ISO Definition
ISO is the sensor sensitivity in digital cameras, in which the signal (light) is amplified; in film cameras, on the other hand, the ISO numbers define “film speed”. The higher the ISO setting, the more sensitive the sensor or film become to light, which can become handy in low light conditions.
The drawback of using high ISO numbers is noise, which makes the picture look grainy. This is especially troublesome in “point and shoot” cameras. DSLR cameras, especially the high end ones, produce a lot less noise than their cheaper counterparts.
The big advantage of digital cameras over the film ones is that the photographer can change the ISO setting whenever he/she wants to. With film, however, the roll must be replaced.