Coal, oil, natural gas (or gas generated from landfills), wood fires, and hydrogen fuel cell technology are all examples of fuels, wherein the resource is consumed to release inherent energetic properties, usually being combusted to generate heat energy. Fuels may be either renewable (like wood or bio-fuel generated from products such as corn) or nonrenewable (like coal or oil). Fuels generally create waste byproducts, some of which can be harmful pollutants.
The Earth generates a lot of heat while going about its normal business, in the form of subterranean steam and magma among others. The energy generated within the Earth's crust can be harnessed and transformed into other forms of energy, such as electricity.
The use of hydropower involves using the kinetic motion in water as it flows downstream, part of the normal water cycle of the Earth, to generate other forms of energy, most notably electricity. Dams use this property as a means of generating electricity. This form of hydropower is called hydroelectricity. Water wheels were an ancient technology which also made use of this concept to generate kinetic energy to run equipment, such as a grain mill.
The sun is the single most significant source of energy to the planet Earth, and any energy that it provides which isn't used to help plants grow or to heat the Earth is basically lost. Solar power can be used with solarvoltaic power cells to generate electricity. Certain regions of the world receive more direct sunlight than others, so solar energy is not uniformly practical for all areas.
Modern windmills can transfer the kinetic energy of the air flowing through them into other forms of energy, such as electricity. There are some environmental concerns with using wind energy, because the windmills often injure birds who may be passing through the region.
Certain elements are able to undergo powerful nuclear reactions, releasing energy which can be harnessed and transformed into electricity. Nuclear power is controversial because the material used can be dangerous and resultant waste products are toxic. Accidents that take place at nuclear power plants, such as Chernobyl, are devastating to local populations and environments. Still, many nations have adopted nuclear power as a significant energy alternative.
Biomass is not really a separate type of energy, so much as a specific type of fuel. It is generated from organic waste products, such as cornhusks, sewage, and grass clippings. This material contains residual energy, which can be released by burning it in biomass power plants. Since these waste products always exist, it is considered a renewable resource.