Freon is a brand name for a group of refrigerants. The most common are R-12, R-22 and R-134a.
Freon was first introduced in 1928 by General Motors as a refrigerant for its AC units. The chemical was so effective that it quickly became standard practice to use this type of refrigerant in all new AC units. It was designed to replace CFCs (chlorofluorocarbons) because they were destroying the ozone layer in the atmosphere. However, since R-22 is a greenhouse gas, it has been phased out for a few years now.
Generally speaking, it is an older refrigerant that was used in older AC systems before being phased out by the EPA due to its ozone-depleting properties. It has since been replaced by R-410A, which uses less energy and doesn’t harm the environment as much as R-22 does.