Film speed is a measurement that indicates a film's sensitivity to light.
ISO is the abbreviation for the International Standards Organization, an agency that specifies many standards, not just photographic ones. Most photographers and film manufacturers refer to films by their arithmetic speed ratings. When the arithmetical speed doubles, the film is twice as sensitive to light, and only needs half the exposure in the camera. For logarithmic speeds, a change of 3 represents either a doubling or halving of the film's sensitivity. Many countries have their own national standards organizations.