A combustion reaction takes place when a fuel and an oxidant react, producing heat or heat and light. The most recognizable form of combustion reaction is flame, with explosions being an even faster form of combustion reaction. A combustion can happen at a wide range of speeds, and can occur in many different environments, but the majority of combustions we know and recognize happen in a fairly limited spectrum.

Often in our world a combustion reaction takes place with a fuel source that is a carbon compound of some sort. Wood is an obvious example of a combustion fuel, as it is burned commonly to create fire for heating spaces or generating heat energy for other purposes. When wood combusts, it is a far from complete reaction, and so leaves behind soot, which is unburned carbon, as well as various carbon compounds. Nitrogen may also be oxidized by the air, creating nitrogen oxides.

Fire burning is an example of a rapid combustion reaction, but it is a somewhat slow reaction nonetheless. Sometimes combustion will be much more rapid, and in the process will also release massive amounts of gas in addition to energy in the form of heat and light. This gas creates a massive pressure shift, which moves the air so quickly that it creates a jarring noise, and we call it an explosion. A rapid combustion may also be used to create usable kinetic energy, as in the internal combustion on a car, or the combustion that takes place in a firearm.

A much slower combustion reaction takes place at lower temperatures, and is not what most people would think of as combustion. Inside our cells, for example, energy brought in as nutrients are converted to adenosine triphosphate and waste in a combustion reaction known as cellular respiration. This is a very slow form of combustion, and so does not appear to burn the way a wood fire does, although fuel is still being converted by an oxidant, and heat is still generated.

Liquid fuels do not actually combust, although it often appears that they do. It is a gas phase of the liquid fuel that is catching on fire, and so can only happen above a certain temperature. The temperature at which a given liquid fuel can ignite is known as its flash point, and is the point at which there is enough volatile fuel in the air to catch fire. For example, alcohol is a liquid fuel, but if you were to take a lower-proof alcohol, sufficiently diluted that there is not much volatile gas in the air at room temperature, it would not ignite. If, however, you were to heat that alcohol up to its flash point, you would find it would quickly ignite.

Different fuels yield different products when they undergo a combustion reaction, some of which are more harmful than others. Carbon ignited with O₂, for example, releases CO₂, 2H₂ ignited with O₂ releases 2H₂O, and CH₄ ignited with 2O₂ releases CO₂ as well. Pure hydrogen, on the other hand, can be used as a fuel as well, and when ignited with oxygen the result is simple water vapor.