Renewable energy relies upon resources that are quickly replenished by natural processes, while nonrenewable energy uses resources that are finite in supply. Common forms of renewable energy include solar, wind and geothermal energy. Fossil fuels and nuclear power are forms of nonrenewable energy.
Renewable energy sources naturally replenish themselves over a relatively brief period. More energy reaches the Earth from the sun in one year than is contained in the world's entire supply of uranium, coal, oil and natural gas, making the supply of solar energy effectively unlimited. Wind and water power result from the natural cycles of the climate and cannot be used up.
Other sources of energy require human intervention to be renewed. Wood can be burned for fuel, but if new trees are not planted to replace those that are cut down, then the supply eventually dwindles to nothing. Similarly, biomass relies upon decaying organic waste as a fuel source. New crops must be planted to renew the supply.
The Earth's supply of uranium is finite. Once it is all used for nuclear fuel, no more can be found. Fossil fuels, such as coal and oil, are produced by natural processes at such a slow rate that the supply is effectively finite.