Fire is a kind-of gas which raises upwards or towards lower pressure/away from higher pressure. It is in Explosives menu. It is yellow, orange or brown depending on it's state and always glows in such colors while in Fire or Fancy Display. It is spawned from menu with a temperature of (422 C: 695K). It is element number 4.
Fire sets flammable things "on fire" by turning them slowly to fire or simply killing them, resulting in what is called burning. Fire also deploys most explosives and makes them explode. Explosives usually are very quickly flammable and their burning also creates pressure.
Fire has many uses, since it is used when materials burn, in most explosives, and also it heats up materials.