Different fire extinguishers combat different types of fires. Using the wrong kind of fire extinguisher can have disastrous results. There are five different types of extinguishers that use four elements to put out fires.
Class A extinguishers combat ordinary combustible material such as paper, wood or plastic. Class B's are used for flammable liquids such as gasoline, grease or oil. Class C fires are those started by electrical equipment, wiring or outlets. Class D fires involve chemicals in laboratories and the extinguishers are specifically made for these types of fires.
Air pressurised water extinguishers (class A) are filled with tap water two-thirds of the way and then filled with pressurised air the rest of the way. Carbon Dioxide extinguishers (class B) are filled with non-flammable carbon dioxide and have no pressure gauge. Dry Chemical (class C) are filled with carbon dioxide and halon gas. Class D extinguishers leave a residue over the surfaces that get sprayed.
Multipurpose extinguishers are able to combat fires from classes A, B and C. They are filled with a dry chemical that works against any of these fire types. The only type they do not fight against are the combustible metals.