The video below gives a brief description of how a fire extinguisher works.
DRY CHEMICAL FIRE EXTINGUISHERS (AB OR ABC Class Fires)
This is probably the most common or at least the most visible type of fire extinguisher a person sees. They are filled with a dry powder or a foam (sodium bicoarbonate or potassium bicoarbonate) which is the extinguishing agent. They are pressurized by an inert (not harmful in these uses) nitrogen gas.
These fire extinguishers can come in a variety of Class combinations. BC or ABC
The BC combinations has the above mentioned extinguishing agent and will leave a mildly corrosive residue which should be cleaned off surfaces as soon as possible.
The ABC type extinguisher is more multi-purpose and is filled with monoammonium phosphate. This is not desirable for computers and other electrical appliances as a yellowish, sticky residue may be left on sprayed surfaces.
A latent advantage of these extinguishing agents is there is a residue which remains on the surfaces sprayed, reducing the chances of re-igniton.
WATER FILLED FIRE EXTINGUISHERS
As is implied, the extinguishing agent is water pressurized by air. These may be referred to as APW extinguishers. (air-pressureized water). Water molecules actually absorb infrared radiation (heat). That is why a pail of water thrown on a camfire does a good initial job of cooling the fire down. (remember to very carefully check the ashes with your bare hands to ensure nothing is left hot and burning) This is also the reason your local fire department relies upon thousands of gallons (or litres) of water from pumper trucks, tank trucks (water tenders) and your local fire hydrants. Remember there are few parts of the average home that are 100% wood.
A major consideration with purchasing and storing a water filled extinguisher is the weather. These type of fire extinguishers can nver be located where freezing temperatures might occur.
AND
You must ensure you never spray water on grease or electrical or Class D fires! The fire will become larger and you may also receive a serious electrical shock.
A quick, true, story
Years ago in a very small rural country town, now far away, the local forest service department (to which I belonged) was requested to respond to a mobile home fire. The mobile home was located on property behind an old service station.
We put our suction (draft) hose into their well and began pumping water onto the mobile home. We soon realized that every time we sprayed the trailer, the flames sort of sparkled and seemed to burn more intensely.
Fortunately it did not take someone very long to realize there must be an old underground gas storage tank which had been leaking and seeping into the well. We immediately stopped spraying the fire. Needles to say, the mobile home was a total loss - but no persons or animals were injured. - and the residents then commented to us that they had always wondered why their water had never tasted very good. ...mmm.
CARBON DIOXIDE (C02) EXTINGUISHERS
The extinguishing agent is carbon Dioxide which is a non-flammable gas. Carbon Dioxide is cold and it is heavier than oxygen. Therefore, the C02 "pushes" the oxygen aside and has a smothering affect on the fire. The cold factor also helps to cool down the heat source. These are good for Class B and C fires. Note also that the carbon dioxide is under very high pressure.
The Carbon Dioxide agent does not leave any harmful residues which makes this type desirable for computers and other electrical equipment fires.
OTHER
The above listed fire extinguisher types are not all there are. These, however, are the most common.
NOTE:
NEVER play with or point a fire extinguisher at a person or animal! A direct spray in the face could cause serious injury.
Remember to determine what class of fire you are attempting to extinguish and do you have the proper type of fire extinguisher for the task?
Do NOT attempt to extinguish a fire until you are certain all persons in the area are going to be safe - yourself included.
CALL 911 OR YOUR LOCAL EMERGENCY SERVICES / FIRE DEPARTMENT FIRST!