Biomass is any organic material that is made by plants and animals. This includes all parts of plants, animal flesh and excretions such as manure. Biomass in rich in energy that can be used for fuel which is called biofuel.
Fossil fuels are also a biomass since it's believed that they come from ancient plants and animals. But usually when people speak of biomass they are talking about material produced from sources that are alive now or were recently.
Biofuel is often talked about as though it was a new technology that we can use in the future to help solve our energy supply problems. But humans have been using biofuel since the beginning of humanity. The food we eat is a form of biofuel that we wouldn't exist without. Humans have also used plant materials such as wood wax and whale oil as a fuel to burn for heat and light for thousands of years.
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Newer technologies have allowed us to use biomass for making other biofuels. Biomass can be burned to power electric generators or made into methane, alcohol or bio diesel for powering our cars. These processes are energy intensive meaning a lot of energy is lost in converting biomass into the other forms of fuel or energy. Because of this these biofuels have not been cost effective or practical.
With all the technology going into biofuel research the one form of biofuel that we have been using since ancient times still remains the most efficient and practical. Burning biomass such as wood for direct heating is still the most efficient.
Instead of focusing on trying to use biomass to fuel our cars and generate electricity we may be 
better off using it to heat our homes and buildings. Firewood may not be practical for most people but pellets made from wood and other biomass may be. Burning pellets is cost effective, clean and may be the most efficient way to utilize the energy in biomass.
For those who want to use biomass as a renewable energy source heating with wood or other biomass pellets are practical and economical and the technology and distribution systems are already available.
to learn how energy can be derived from biomass.