Water is the most familiar extinguishing material, and it is one of the most effective. But it can be dangerous in the wrong situation. A water extinguisher can put out things like burning wood, paper or cardboard, but it does not work well on electrical fires or fires involving inflammable liquids. In an electrical fire, the water may conduct the current, which can electrocute you. Water will only spread out an inflammable liquid, which will most likely make the fire worse.
One popular extinguisher material is pure carbon dioxide. In a carbon dioxide extinguisher, the carbon dioxide is kept in pressurized liquid form in the cylinder. When the container is opened, the carbon dioxide expands to form a gas in the atmosphere. Carbon dioxide gas is heavier than oxygen, so it displaces the oxygen surrounding the burning fuel. This sort of fire extinguisher is common in restaurants because it won't contaminate the cooking equipment or food.
The most popular extinguisher material is dry chemical foam or powder, typically made of sodium bicarbonate (normal baking soda), potassium bicarbonate (nearly identical to baking soda), or monoammonium phosphate. Baking soda starts to decompose at only 158 degrees Fahrenheit (70 degrees Celsius), and when it decomposes, it releases carbon dioxide. The carbon dioxide, along with the insulation of the foam, works to smother the fire.
Most fire extinguishers contain a fairly small amount of fire-suppressant material -- you can use it all up in a matter of seconds. For this reason, extinguishers are only effective on relatively small, contained fires. To put out a larger fire, you need much bigger equipment -- a fire engine, for example -- and the professionals who know how to use it. But for the dangerous flames that can pop up in your house, a fire extinguisher is an invaluable lifesaver.
To learn more about fire extinguishers and other fire-fighting equipment, check out the links below.