Fire is the combustion of wood and other organic materials.  Heat and light always exist during organic fires, but light may not be created by some chemical fires.  Organic fires require oxygen.  

Combustion is the rapid chemical reaction between oxygen and the material in the wood.  After burning, all that is remaining is ash, which is carbon.  Smoke is the visible release of the chemicals leaving the fuel.  Its pretty simple.

I strongly recommend going here to learn more - 
http://www.fire.org.nz/engineer/science/science.htm