Use Class A fire extinguishers to put out fires fueled by ordinary combustibles such as wood or paper.
Use a Class B fire extinguisher for flammable liquids. This would include fires caused by grease, gasoline or oil.
Class C fire extinguishers should be used on electrical fires. The material inside these extinguishers is non-conductive and specially designed for this type of fire.
Most ordinary household applications will not require a Class D fire extinguisher. These are used to put out flammable metal fires, such as magnesium, sodium-potassium alloys or powdered aluminum. Factories working with these types of metals should be equipped with the proper fire extinguishers.
When attempting to use a fire extinguisher, always make sure you have easy access to an exit. If the fire begins to spread, do not attempt to put it out yourself. Leave the building as fast as possible and wait for the fire department.
Never try to fight a fire if you don't know what is burning. Using the wrong kind of fire extinguisher can actually make the fire spread faster.