ISO (Film Speed) explained

ISO or Relative Speed of the Sensor

The Past

Two major scales have been used for letting us know what the relative light gathering speed film had. DIN (German, Deutsches Institut für Normung), and ASA, (American Standards Association)

The two major scales to standardize the Film speed world wide was combined to produce the ISO (International Standard Organization).

From Wikipedia

The American National Standards Institute or ANSI (pronounced /ˈænsiː/) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international standards so that American products can be used worldwide. For example, standards make sure that people who own cameras can find the film they need for that camera anywhere around the globe.

ANSI accredits standards that are developed by representatives of standards developing organizations, government agencies, consumer groups, companies, and others. These standards ensure that the characteristics and performance of products are consistent, that people use the same definitions and terms, and that products are tested the same way. ANSI also accredits organizations that carry out product or personnel certification in accordance with requirements defined in international standards

The Present

ISO Standard is the current standard that Camera and Film makers use to assign the numeric value.

The numbers are set to double or 1/2. IE: ISO 100 to ISO 200, ISO 200 is twice the speed as ISO 100. or Visa Versa.

The makers adjust the gain on the sensor to achieve this and add a software scale to allow you pick the Sensor speed you want.  Read more here.

More Links of Intrest

DIN  Wikipedia

ASA Wikipedia