Fire refers to the combination of heat and light given off in the form of flames when a fuel source is consumed. Essentially, fire is made possible by a chemical reaction between oxygen in the atmosphere and a fuel source (i.e. wood or gasoline). In order for this reaction to happen, the fuel source must reach an ignition temperature, which provides the system with enough activation energy for combustion to occur. Reaching this ignition temperature can be achieved through a match, a beam of light, friction, or lightning.
Products of this combustion reaction include heat, light, and a few other byproducts. As mentioned, the heat and light are released in the form of flames. Variation in color throughout these flames is due to differences in temperature. The hottest part of the flame (at the base) is blue in color, while cooler areas at the top of the flame appear orange or yellow.
Fire is often noted as being continuous and difficult to put out when out of control. It is for this reason that fires were able to keep our ancestors warm all night, but it is also because of this quality that wildfires, lightning strikes, and campfires gone awry can be devastating. This continuous, never-ending quality of fire is due to the self-perpetuating nature of the combustion reaction. The reaction releases heat, which keeps the fuel source and oxygen at the ignition temperature, thereby allowing the reaction to continue proceeding.