Fire extinguishers, which are effective at controlling small fires, work by removing the oxygen needed to maintain the combustion process.
Fundamentally, the principle behind putting out a fire involves removing one or more of the components required to sustain the reaction, either the heat, the oxygen or the fuel. But removing the fuel is generally the most difficult thing to accomplish, with the exception of a forest fire where creating a fire break of cleared or scorched land to the stop the growth of the fire is a good response. It’s more practical to remove other components that contribute to a blaze.
Most fires are most effectively dealt with by removing the oxygen or the heat or both. Fire fighters routinely spray water on burning structures since water is available in large quantities and its high heat of vaporization helps to absorb some of the heat of the fire. Unfortunately, some types of fires, such as grease fires, do not react well with water, which prompted the development of fire extinguishers.
Fire extinguishers that work by blanketing the fire with a gas or powder to eliminate available oxygen are designed to put out small scale fires before they grow too large. They are rated according to the types of fires on which they may be used on effectively, and it is critical to choose the correct equipment for a given fire. A/B/C fire extinguishers containing a material such as monoammonium phosphate are appropriate for use on fires fueled by paper or wood (Class A), as well as electrical and chemical fires (Classes B and C respectively), so this equipment is frequently found in homes. Class D fire extinguishers are appropriate only in rare environments where flammable metals are used, since fires involving metals such as potassium or sodium are not halted by the A/B/C fire extinguishers.
B/C fire extinguishers often contain sodium bicarbonate powder which forms carbon dioxide in the heat of the fire. Such fire extinguishers are frequently used in laboratory settings. Pressurized carbon dioxide is also used in B/C units and is the preferred material for extinguishing flames on the clothes of Hollywood stuntmen and for areas where particles from dry chemical fire extinguishers would potentially damage electronic and computer equipment. Sadly for enterprising beer-drinkers, the carbon dioxide smothers the fire by removing the oxygen rather than by cooling it, so using a fire extinguisher to chill beer rapidly is not effective.
More information on fires and fire extinguishers is available at http://home.howstuffworks.com/home-improvement/household-safety/fire/fire-extinguisher.htm