Fire is the superheating of flammable materials. After a certain temperature all things will release what is known as passives. Passives are things like water and oxygen that bond on an atomical level to an item. Once these unburnable passives are released there is only fuel left to burn. This state is call pyrolysis. Pyrolysis is when an item is so superheated it begins to give off flammable gasses. These gasses then ignite and produce a flame which is what we commonly refer to as fire. Fire is maintained not by simply heat but by the release of these flammable gasses fueling the fire to burn longer and hotter. As long as the fire has Heat, Oxygen and Fuel; it will burn.
A flashover is the stage where the total thermal radiation from the fire plume, hot gases and hot compartment boundaries causes all exposed combustible surfaces to Pyrolyse and ignite when there is adequate ventilation. This sudden and sustained transition of a growing fire to a fully developed fire is known as a flashover. A flashover can be very sudden. It can quickly escelate an undercontrol fire into a fully involved structure fire. All firefighters are given training to understand the symptoms of a flashover and are taught how to stop a flashover from compromising the structure of a propery as well as the safety of all firefighters involved in the scene.
A backdraft is where limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and unburnt pyrolysis products. If these accumulate, the admission of air when an opening is made to the compartment can lead to a sudden deflagration. This deflagration moving through the compartment and out of the opening is a backdraft. The force of a backdraft has the potential to damage building elements resulting in an unstable structure. Backdraft can move upwards of 80mph and is a literal fireball. Backdraft making contact with a firefighter would very likely result in the loss of life. Backdraft is a very serious danger faced in the world of firefighting and all firefigthers on a structural scene are expect to take great care to notice the signs and symptoms of a backdraft.