ISO? what is it....

Film speed is the measure of a photographic films sensitivity to light, determined by sensimentry and measured on various numerical scales, the most recent being the ISO system.
A closely related ISO system is used to measure the sensitivity of digital imaging systems.
Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the same image density as a more sensitive film, and is thus commonly termed a slow films. Highly sensitive films are correspondingly termed fast films.