Biomass is renewable organic matter that can be used as fuel. It is living or recently dead material - wood and other plant matter, or even animal waste. Fuel derived from biomass is known as biofuel. It does not include fossil fuels, which have formed - and stored their carbon - over millions of years. Because the CO2 released when biomass is burned is balanced by the CO2 absorbed during its production - and because new plant matter is re-growing and absorbing more CO2 all the time - it's regarded as carbon neutral.