"Fire is the rapid oxidation of a material in the exothermic chemical process of Combustion, releasing heat,light and various other products", defines Wikipedia. But Fire has got more than that to be known.
We can feel the heat when a piece of wood is burnt. Also sounds can be heard from it.Getting too close to the fire can be harmful for ourselves .But as it is said ,Fire is one of the greatest discovery of mankind. Here's what Greek mythology has to say about the human usage of fire.
     Before the creation of humanity, the Greek gods won a great battle against a race of giants called the Titans.Most Titans were defeated or driven to the eternal hell of Tartarus.But a Titan named Prometheus, along with his brother Epimetheus , fought from the side of the Gods.Happy with them Zeus ,the god of gods ,provided them with the honorary task of creating living organisms down in Earth.Epimetheus formed animals and gave each of them a special skill and form of protection. Prometheus took his time molding man and was left with no forms of protection since Epimetheus had already given them all away. Prometheus knew that man needed some form of protection and asked Zeus if he could let man have fire.Zeus refused.Fire was the only for the gods.Prometheus ignored Zeus and gave man fire anyway.For this grew back everyday so that Prometheus would have to endure torture everyday. Anyway that's a myth.But ever after the discovery of fire back about 4 million years ago,human civilization has never looked back.
Fire is considered to be first controlled by mankind some 1.5 million years ago.But researchers also prove that the use of fire ,by the then species of 'Homo Erectus',some 4 million years ago.The mastery over fire provided mankind ,who had almost no self defense tools,to not only survive but also conquer over the world.
And again, What is Fire actually? Fire is certainly not a substance in liquid phase let alone solid.Fire doesn't seem to be gas either as it gets localized only to the source of fuel unlike gas for which diffusion is an intrinsic property.Fire doesn't contain gas molecules.Rather it doesn't contain molecules like a physical matter.And then,contrary to the popular misconception, fire is not something in plasma (fourth states of matter ) state either.I'm plasma the atoms lose all their electrons leaving a highly electrified cluster of nuclei and free electrons. But fire cannot be in plasma state - because plasma doesn't exist in stable form in our earth.They only formed when electrons are ionized at ultra high temperature but fire flames are seen only at some hundred degrees. 
Fire is actually the front of the oxidation reaction of a gas or vapour in atmosphere. It is usually identified by flames which are the visible part of the reactions creating a zone of visible light in a single or mixture of tapered shapes.Flames of fire have various temperature zones .Usually the hottest slightly above the centre of the flame due to the highest concentration of gaseous or vaporized fuel below the center.At a certain point in the combustion reaction called the ignition point ,the flames are produced.Flames consist of carbon-dioxide,water vapour, oxygen and nitrogen as gaseous fuel.Fire emits heat as well as ligy.For fire to take place ,three things are necessary - fuel,oxygen and energy (generally in the form of heat).
To hit the bull's eye ,Fire is our sensory experience of a chemical reaction called combustion. Fire stimulates a lot of our senses at the same time so that it makes us create a certain scenario we expect to come from a physical thing.