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.
DIN is the abbreviation for the Deutsche Industrie Norm, a German standards organization. These organizations have standards that also specify how film speed should be measured, hence ASA, ANSI, or DIN film speeds. Thus the ASA and ANSI film speeds can be regarded as interchangeable with ISO numbers. Originally, the DIN standard's body only specified films using a logarithmic system, but the current DIN standard now includes arithmetic speeds. While this appears strange, it really is very logical.