ISO is a "Standard" and the acronym stands for "International Standards Organization." In relationship to film it spells out a commonality between films as to how much light is required for a quote "normal exposure." Light and time both play a part. To understand this you have to know that the effects of light on film emulsion is cumulative. That is to say the longer you expose the light sensitive material to light, the more cumulative the effect becomes. This is not as apparent with digital photography due to the inherent nature of the media to compensate for the amount of light in a given situation automatically. With film in a low light situation, the photographer must either select a film with greater sensitivity to light, or, must expose the film for longer periods to ensure that an adequate amount of light reaches the film emulsion to capture the intended image.
