Fire is the result of a chemical combustion reaction, typically a reaction between oxygen in the atmosphere and some sort of fuel (wood or gasoline, for example). Of course, wood and gasoline do not spontaneously catch on fire just because they are surrounded by oxygen. For the combustion reaction to take place, the fuel has to be heated to its ignition temperature.
When you ask a professional, like a scientist or firefighter, their definition of fire is much more precise. Scientifically speaking, fire is the result of a chemical reaction called combustion. When combustion occurs, substances like wood, oil, coal, paper etc. combine with the oxygen in the air to generate carbon dioxide, water and waste gases, along with an enormous amount of heat.
About Fire Extinguishers:
Before you can understand how fire extinguishers work, it is helpful to understand how fire itself works. Fire results from a chemical combustion reaction that requires three things: intense heat, oxygen, and fuel, such as wood or gasoline.
Fire extinguishers work by removing one or more of those three necessary elements of fire. This usually occurs by applying a substance that either cools the burning fuel or displaces the oxygen surrounding the fuel.
Most fire extinguishers consist of a metal cylinder filled with either water or a dry chemical foam or powder that acts as a smothering agent. They work much as an aerosol can. When you depress a lever, a propellant expels the material inside under high pressure.
When using a fire extinguisher, you should always direct the spray of water, foam, or powder toward the fuel source, not the flames. If you can cool the fuel source or cut off its supply of oxygen, the fire will die out.
Fire extinguishers are best used for small fires that can be easily contained. This is because most fire extinguishers contain only a limited amount of water, foam, or powder, which can be used up in seconds when it is expelled at high pressure. Larger fires require bigger equipment and the professionals that know how to use it.