Importantly, these are for use against Class A (that is, combustible) fires, and inappropriate for other classes.
Class B fires involve alcohol, gasoline, diesel oil and flammable gases. Carbon Dioxide and Dry-Chemical agents, Foam and Halons Extinguishers form suitable extinguishing agents, lowering the temperature below flash point, by removing the fire's oxygen supply. Two of these agents are suitable too, for use on Class C fires involving energized electrical equipment, with the important exception of foam and water as these methods conduct electricity.
Class D fires involve combustible metals such as sodium, potassium and zirconium. Hazardous in powdered form, burning at high temperature, water is ill-advised, due to its possibly acting as an explosive rather than extinguishing agent. No single extinguishing agent exists for all metals, though dry-powder compounds are recommended, commonly, Lith-X and Meth-X.
Class K fires involve vegetable and animal oils, and fats in appliances in kitchen restaurants and cafeterias. Extinguishers use saponification (soap converting agents), on hot grease and dry chemicals.