Biomass fuel is living, or recently living material, used for fuel. Biomass fuel is most commonly derived from plants, for example using dead tree branches for cooking fires, or the waste from timber yards and cultivated palms (palm oil) as a fuel source for industry.
Fossil fuels come from ancient plant and animal remains, but, because the carbon they contain has been "locked up" and not available to the carbon cycle for millions of years, they do not fit the definition of biomass fuels. When fossil fuels are burnt, this "extra" carbon is released, giving a large "carbon footprint". Wood, when burnt, releases no more carbon dioxide into the atmosphere than it would if it just decomposed naturally in the carbon cycle, so is "carbon neutral".