There is really no universally agreed upon definition for green energy. Various interpretations of green energy exist. The most expansive one includes all renewable energy sources, nuclear power and reformulated fuels. The common theme being sources of power that do not harm the environment.
Renewable Energy Sources
Renewable energy sources are in continuous, but interrupted, supply. They are derived from water, the wind and the sun. Hydroelectric power plants have been around since large rivers could be dammed and water flow controlled to drive a turbine generator. If humans vanished from Earth, the Hoover Dam would continue to produce electricity for some time until engineered parts failed without the regular maintenance done by humans. Wind has been used since early history to grind grains. Modern wind turbines are now in use to produce electricity to power our homes. Passive solar power was implemented by the early Greeks and Romans in their architectural designs to heat and cool buildings. Solar panel systems have evolved to actively harness the sun's energy and convert it into electricity to power our electronics, appliances and lights.
Water, wind and the sun all are renewable sources of energy or "green" energy; we cannot run out of the wind, but the wind does not blow consistently. It is generally windier at night when electricity demand is low. Renewable energy requires excellent storage capacity in the from of large battery banks in order to mesh with our current electrical system and provide consistent power.
Nuclear power generates electricity from heat that is created when atoms are split; the heat is used to create steam which runs a steam generator. Splitting atoms creates radioactive waste and excessive heat which must be controlled to prevent catastrophic consequences. Nuclear power can be considered green energy because no greenhouse gases are produced through its process. Radioactive waste is problematic because it takes hundreds of thousands of years for it to dissipate. Disposal of radioactive wastes requires that it be contained for a long time to prevent harmful levels from reaching humans, plants and animals.
Global electricity demand is so high that to truly decrease greenhouse gas emissions, including carbon dioxide, nuclear power supplies would have to be increased. Only with increased nuclear power could fossil fuel use be phased out and global warming avoided. For this reason, nuclear power can be considered part of the green energy definition.
Reformulated fuels include biofuel and biogas. Ethanol is made from corn and can be added to fossil fuels to run automobiles and trucks. Other sources of biofuel include vegetable oil, animal fats and algae, all of which can be converted to fuel through chemical reactive processes. With the exception of algae (which needs a lot of carbon dioxide to grow), these reformulated fuels do produce greenhouse gas emissions, but at much lower levels than fossil fuels. They are considered green energy sources because the rate of global warming would slow down with their use.
Biogas results when garbage and animal waste decompose in an anaerobic environment. Naturally occurring microbes eat the organic material and produce methane gas. While methane is a greenhouse gas, it is also the major component of natural gas. Methane can be used by itself to run simple generators or reformulated to power a natural gas power plant. It is considered a green energy source because it reuses a greenhouse gas thereby offsetting other fossil fuel use. Also, by offsetting fossil fuel demand it alleviates damages incurred when gas and oil are extracted from the Earth, transported and refined for use.
Green energy, then, is any source of power that is sustainable and not excessively harmful to human health or the environment. A strict definition would include only water, wind and solar power. A more expansive definition would include nuclear power, biofuel and biogas.