Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat, smoke & electromagnetic radiation (light).

Four things must be present at the same time in order to produce combustion:

Fuel
Any combustible material - solid, liquid or gas. Most solids and liquids must vaporize before they will burn.
Fuel
An oxidant (usually oxygen) - Sufficient oxygen must be present in the atmosphere surrounding the fuel for combustion.
Heat
Sufficient heat energy must be applied to raise the fuel to it's ignition temperature
Chemical, exothermic reaction
This reaction can occur when all three of the above elements are present in the proper conditions and proportions. Fire (rapid oxidisation) is the result of this chemical reaction.
Fire Triangle

The combination of these three elements is frequently referred to as the "fire triangle". Add in the fourth element, the chemical reaction, and you actually have a fire "tetrahedron." The important thing to remember is that if any one of these four things can be removed there will be no fire or the fire will be extinguished.

Fires start when a flammable or combustible material with adequate supply of oxygen or other oxidizer is subjected to enough heat.

Fire is extinguished when any of the elements of so-called fire triangle (heat, oxygen or fuel) are removed. The unburnable solid remains of fire are called ash.

Flames can conduct electricity, as a small portion of any fire is ionized. This has been demonstrated in the laboratory and also in large wildfires that occur in the vicinity of power lines. This ability to conduct electricity is due to its partially plasmatic nature.

Four Stages of Combustion

Incipient Stage

The incipient stage, which may last for seconds to days, there is no noticeable smoke, heat or flame. During this stage, flammable gasses, or “products of combustion” are emitted.

Smouldering Stage

The smoldering stage, during which there still is no substantial flame or heat, but the combustion increases enough to create visible smoke.

Flame Stage

The flame stage usually involves less smoke, but flames break out generating substantial heat.

High Heat Stage

The fourth stage of a fire is often referred to as the high heat stage. At this point, the fire has spread rapidly throughout the home, producing extensive flames, extreme heat and many toxic gases.

Heat

Heat is the process of energy transfer to a body in any other way than due to the work performed on the body!

Energy transfer can occur between objects through one or more of the following - radiation, conduction and convection.

The transfer of heat is normally from a high energy body to a lower energy body. 

Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics.

Temperature is a measure of the internal energy of a body.

Conduction, Convection & Radiation...

Conduction
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole.
Convection
Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.
Radiation
Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object.

Smoke

Smoke is an aerosol or mixture of particulate materials suspended in a gas (air) that comprises a collection of airborne solids, liquid particulates and gases emitted when a material undergoes combustion...

Smoke particulates vary in size.

Depending on particle size, smoke can be visible or invisible to the naked eye.

Electromagnetic Radiation

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

Light is a form electromagnetic radiation of a wavelength that is visible to the human eye. Flames are a form of visible light.