A natural resource qualifies as a renewable resource if it is replenished by natural processes at a rate comparable or faster than its rate of consumption by humans or other users. Solar radiation, tides, winds and [hydro energy] are perpetual resources that are in no danger of being used in excess of their long-term availability. The term also has the connotation of sustainability of the handling of waste products by the natural environment.
Some natural renewable resources such as geothermal, fresh water, timber, and biomass must be carefully managed to avoid exceeding the environment's capacity to replenish them. A life cycle assessment provides a systematic means of evaluating renewability.
Solar power is the energy derived directly from the Sun. It is the most abundant source of energy on Earth. It is captured by photovoltaic cells, or by using sunlight to heat water. The Sun will continue to shine for about 5 billion years.
Wind power is derived from uneven heating of the Earth's surface from the Sun and the warm core. Most modern wind power is generated in the form of electricity by converting the rotation of turbine blades into electrical current by means of an electrical generator. In windmills (a much older technology) wind energy is used to turn mechanical machinery to do physical work, like crushing grain or pumping water.
Hydropower, energy derived from the movement of water in rivers and oceans (or other energy differentials), can likewise be used to generate electricity using turbines, or can be used mechanically to do useful work.
Geothermal power directly harnesses the natural flow of heat from the ground. The available energy from natural decay of radioactive elements in the earth's crust and mantle is approximately equal to that of incoming solar energy.
Alcohol derived from corn, sugar cane, switchgrass, etc. is also a renewable source of energy. Similarly, oils from plants and seeds can be used as a substitute for non-renewable diesel. Methane is also considered as a renewable source of energy.
Techniques in agriculture which allow for minimal or controlled environmental damage qualify as sustainable agriculture. Products (foods, chemicals, biofuels, etc) from this type of agriculture may be considered "sustainable" when processing, logistics, etc. also have sustainable characteristics.
Water can qualify as a renewable material when carefully controlled usage, treatment, and release are followed. If not, it would become a non-renewable resource at that location. For example, groundwater could be removed from an aquifer at a rate greater than the sustainable recharge. Removal of water from the pore spaces may cause permanent compaction (subsidence) that cannot be reversed.