Film speed refers to the sensitivity of film to light. In general, films with a higher sensitivity (larger number, fast) have coarser grain and do not register detail or reproduce enlargements as well as films with lower sensitivity (lower number, slow). Slower film speeds equate to better detail and sharpness in your photographs, but need to be exposed for longer than faster film speeds. Film sensitivity is measured by a set of standards established by the American Standards Association (ASA) and the International Standards Organization (ISO). For all practical purposes, ASA and ISO numbers are interchangeable. A typical number series for film speed, from slow to fast, is:
2550100200400800
Each stop on this scale is twice as sensitive as the number to the left on the scale, and half as sensitive as the number to the right.
When using a light meter, set it to the same number as is specified on the film to get the correct exposure readings. Vintage cameras will feature DIN and ASA, while modern cameras use ISO. The DIN system was defined by the German standards body (Deutsches Institut für Normung) and was published in January 1934. It was originally only meant for black-and-white negative film and, along with ASA, was superseded in the 1980s by ISO.