What exactly is fire?

Fire is the oxidation of a combustible material releasing heat, light, and various reaction products such as carbon dioxide and water.[1] If hot enough, the gases may become ionized to produce plasma.[2] Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity might vary. Fire in its most common form can result in conflagration, which has the potential to cause physical damage through burning.

when a combustion reaction takes place bonds between elements are broken and reformed into new more stable bonds. because these bonds are more stable they hold less energy. so where does all this energy go? its transformed into light and heat. the light part is what you see and call fire. electrons become exited and jump up a energy level. when they return they give off their energy as a photon. lots of photons are being shot out from the reaction. the more energy at a specific place the more energy the photons have. the differences in energy changes the shape of the wave the photons make. this causes the color change you observe when you look at a flame. the closer the fire is to the reaction the "hotter" colors you see.

It's the characteristics. Oxygen, and whatever chemicals are in your Fuel source, (could be a million different things.) And of course Heat.

It's mainly whatever chemicals are involved in the characteristics to cause the chain reaction.

fire is a chemical reaction

It is an example of the state of plasma