Firefighters are taught the three major classes of fires: ordinary combustibles, such as wood, cloth, and paper (Class A); flammable liquids, such as gasoline and oils (Class B); and energized electrical fires, such as panel boxes and appliances (Class C).

That's important to you, the homeowner, because fire extinguishers are rated by the types of fires they extinguish. You should match the location and potential fire type with the proper extinguisher. Never try to use an extinguisher on a class of fire it's not rated for. That can actually cause you to get hurt or the fire to spread.

Most consumer fire extinguishers can put out more than one type of fire. For example, an AB class extinguisher could put out paper and grease fires in a garage, but couldn't extinguish an electrical fire (because the extinguishing agent conducts electricity). On the other hand, ABC and BC class extinguishers can put out every type of fire you'd encounter, making them particularly useful in the kitchen.

Fire extinguishers are also rated by their capacity -- that is, the size of the fire they would extinguish. The higher the number before the class, the greater the capacity. For example, a 10B:C extinguisher would cover twice as much area as a 5B:C.

Just like your smoke detector, your fire extinguisher needs to be maintained.