Foam fire extinguishers are the advanced kinds of the fire extinguishers that helps in controlling the fire caused by oil and other flammable things which cannot be extinguished with the help of water. These extinguishers have the chemicals that are stored in form of liquid under pressure and thus when one breaks the seal, the pressure forms the foam. There are many chemicals that are used in the foam fire extinguishers, few of them are the AFFF (aqueous film forming foam), AR-AFFF (Alcohol-resistant aqueous film forming foams) and many others. As the name suggests these foams form a film or a layer over the burning surface.
The foam fire extinguishers are useful because they can be used over oil and other petroleum burns as well, the foam released by the extinguishers covers the surface completely and cools the surface down. As the foam forms a coating over the burning surface the ignition temperature of the burning product in reduced and thus the fire stops. The chemicals used in the foam fire extinguishers cool the surface helps to control the fire as they have the chemical property of cooling the surface they are spread on, this property helps to control fire.