Camera film is basically a flexible ribbon, coated with chemicals that react to light. A section of this ribbon, placed behind a lens, can record an image as it reacts. How quickly it reacts to light is described by the film speed. The higher the ISO number, the faster the reaction. 

But, before ISO standardized film speeds for colour film in 1979, there were a number of competing systems. The best known of these was developed by the American Standards Association, and the letters ASA, are often found on vintage cameras from the 1950's to the 1970's. Incidentally the ASA, went on to change its name, in 1969, to ANSI, the ISO member for the USA.

What do the numbers mean?

The ASA system was superseded by the ISO film speed standards, such as ISO 6, ISO 2240, and ISO 5800, but the linear scale that it established for describing film sensitivity remains to this day. So, a fast reacting film would typically be used in conditions where there is less light, or for fast moving objects. Because the camera has less light, or less time to capture the image. For brighter conditions, or for static and slow-moving objects, a slower film works best. Slower films, those with lower ISO numbers, for example, ISO 200 or ISO 400, tend to produce clearer pictures. The measured pace of the reaction produces a clear, strong image (provided that the photographer has also selected the right duration, and size, of opening of the aperture through which the image is projected onto the film). Whereas a faster, more reactive film used under the same conditions is likely to produce an image that is grainier, less clear, slightly speckled. Photographers call this lack of clarity 'noise'. That's why film speed selection is an important element in taking a good picture with a film camera.

Still in use today

The concepts of 'ISO speed', 'aperture', 'exposure' time have all been carried across to digital photography, although many models including most smart phone cameras, will sort these out for you. Professionals and enthusiasts tend to pay attention to these parameters, whereas the rest of us are happier to just point and shoot, relying on digital filters or photo retouching skills to get the effect that we're looking for.

While technology has moved on, with many of us carrying powerful digital cameras with us everywhere that we go, the underpinnings of turning fleeting moments into lasting memories are underpinned by some of ISO's best-known standards.