Biomass can be used to produce renewable electricity, thermal energy, or transportation fuels (biofuels). Biomass is the only source of fuel for domestic use in many developing countries. Biomass is all biologically-produced matter based on carbon, hydrogen, and oxygen. Unlike other renewable energy sources, biomass can be converted directly into liquid fuels - biofuels - for our transportation needs (cars, trucks, buses, airplanes, and trains). The two most common types of biofuels are ethanol and biodiesel. Ethanol is mostly used as a fuel additive to cut down a vehicle's carbon monoxide and other smog-causing emissions. The energy stored in biomass can be released to produce renewable electricity or heat. Bio-power can be generated through combustion or gasification of dry biomass or biogas (methane) captured through controlled anaerobic digestion. Co-firing of biomass and fossil fuels is a low-cost means of reducing greenhouse gas emissions, improving cost-effectiveness, and reducing air pollutants in existing power plants. A number of transportation fuels can be produced from biomass, helping to alleviate demand for petroleum products and improve the greenhouse gas emissions profile of the transportation sector.