Class A Extinguishers
Fire extinguishers in the Class A category have a green symbol on the front of the canister with a picture of a burning garbage can. These extinguishers are used on ordinary combustibles, such as wood or paper, and typically contain plain water.
Class B Extinguishers
Class B fire extinguishers have a red square as their symbol containing a picture of a gasoline can with a burning puddle inside. These are used for flammable liquids such as gasoline or oil. They typically have carbon dioxide or foam inside.
Class C Extinguishers
Class C is used for energized electrical equipment that has caught fire, such as a fuse box or an outlet. These have a blue circle with a picture of an electric plug and a burning outlet as their symbol. They typically contain a dry chemical to put out these fires.
Class D Extinguishers
Class D extinguishers are usually only found in laboratories. They are for use on chemicals that have caught fire, such as magnesium or titanium. The symbol for this class is a yellow star that contains no added picture.
Read more: Why Are the Fire Extinguishers Different Colors? | eHow.com