Biomass might literally mean a biological mass, but it actually denotes sources that are burned to produce energy. Biomass is considered a renewable energy source. The primary one that has been used for thousands of years is wood; it is still widespread in many primitive areas and it is burned in old and new burners. Biofuels like ethanol from crops is another important source that has been on a rise. They are cheap and they are used a lot in developing countries. Finally, the burning of municipal waste is another source and many Nordic countries use it to produce energy and even some import waste to use as fuel.