GREEN ENERGY - "Green Energy" is typically energy which when used, results in very little or no GHG (Green House Gas) Emissions. Green House Gasses are gasses which are deemed responsible for the "Green House Effect" or the warming of the Earth's Atmosphere.
CO2 (Carbon Dioxide) is usually the focus when people talk about Green energy. In Europe the EUETS (European Emissions Trading Scheme) is a scheme which is attempting to reduce the amount of CO2 emissions in the European Union through a "cap and trade" system. Other Green House gasses are Methane, nitrous oxide, ozone and chlorofluorocarbons.
Renewable Energy is generally thought of as "green" because energy of this type comes from sources that can be replenished relatively easily as opposed to fossil fuels which are effectively depleated once you use them. Crops such as Corn, Rapseed and soyabeans can all be used to produce energy. These agricultural sources of energy can of course be replanted again after harvest Solar, wave and wind energy are also thought to be "green" sources of energy. It is important to note that when considering how "green" a source of energy is it might be prudent to think about the entire process around how that source of energy is produced. Take nuclear power for example. Although the actual nuclear plant itself may have no carbon emissions, the mining and transportation of the Uranium needed to produce Nuclear power results in a fair amount of CO2 production. The mining and transport of coal for a coal powered plant is no different. It is very difficult and takes some careful thinking to produce truly "Green" Energy.