Biomass is biological material derived from living or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material.
Fossil fuels such as coal, oil and gas are also derived from biological material, however material that absorbed CO2 from the atmosphere many millions of years ago.
Biomass has achieved global acceptance as a long term renewable replacement for fossil fuels, it is a highly efficient form of energy conversion (especially relative to coal) and its use for power production contributes towards the reduction of greenhouse gasses due to its “carbon neutral” status, which due to the fossil fuel displacement value of the power produced, generates positive carbon emissions credits
As fuels they offer high energy density, but making use of that energy involves burning the fuel, with the oxidation of the carbon to carbon dioxide and the hydrogen to water (vapour). Unless they are captured and stored, these combustion products are usually released to the atmosphere, returning carbon sequestered millions of years ago and thus contributing to increased atmospheric concentrations.