Dry Chemical extinguishers are usually Multi-Class rated, and contain an extinguishing agent and a compressed, non-flammable gas as a propelllant.
Halon extinguishers contain a gas that interrupts the chemical reaction taking place as fuels burn. These extinguishers are often used to protect valuable electrical equipment as they leave no residue to clean up. Halon extinguishers have a limited range (4 to 6 feet) and the intial application should be made at the base of the flames, even after the flames have been extinguished.
These extinguishers contain water and compressed gas, and should only be used on Class A (ordinary combustible) fires.
Carbon Dioxide (CO2) extinguishers are primarily effective in extinguishing Class B and C (liquids and electrical) fires. Since the gas dissipates quickly, they only have an effective reach of 3 to 8 feet. The carbon dioxide is stored as a compressed liquid, and as it expands it cools the surrounding air, often causing ice to form on the "horn". Since the fire could re-ignite, application should continue even after the fire is out.