Energy is “Renewable” if it is naturally replenished. Thus hydro, wind, solar, wave and tidal energy are all considered renewable.
Conversely, oil, gas and coal generating stations all consume fossil fuels, which are a finite and limited resource.
Even renewable energy use has environmental impacts. For example, wind turbines kill birds, have a large visual impact and produce noise at levels that can affect health. See Environmental impacts to see the impacts of hydro power. The most important design measure, is not to impound (fill) hydro reservoirs without first logging and clearing the reservoir area. Forest that is flooded and left to rot underwater produces methane which is 40 times more harmful to the ozone layer than carbon dioxide.