All household extinguishers are classified A, B, or C (or a combination of these) on the label to indicate which types of fires — ordinary combustibles, flammable liquids, or electrical—you can use them on. Many of the ones sold at home stores are classified A:B:C and fight all three types of fires.
The main distinction among home extinguishers is size. In most cases bigger is better, but sometimes the biggest extinguishers are too heavy to maneuver. (The weight on an extinguisher refers to the amount of chemical inside; the canister adds several more pounds.) There's also a difference between rechargeable extinguishers, with metal valves, and disposable ones, which have plastic valves. A rechargeable one will cost more, but refilling it once the pressure gauge shows that use or time has depleted the contents is still less expensive than buying a new disposable one.
The National Fire Protection Association (www.nfpa.org) recommends an extinguisher for each floor. But no matter how many you have, nothing can substitute for the most important safety tool: a fire plan. Make sure everyone in the family knows how to get out in a hurry, where to meet outside, and how to call 911. Even if you think you've put out the fire on your own, don't cancel that emergency call. Leave it to the pros to decide if it's really out.