Renewable energy resources are sources of energy that create power without depleting a natural resource. Usually, this means that energy can be harvested in such a way that the resource itself is not used up or destroyed. In certain cases, this means that there is so much of the resource that use of the resource is not problematic because it will never run out. Among the most commonly known renewable energy resources, solar energy, wind power, and geothermal energy are popular. There may be other renewable energy resources about which we do not know or that have not yet been successfully converted into energy.
When most people think of renewable energy resources, they think of things like solar power and wind power. In these cases, natural resources like wind and sunlight are converted into energy that can be used in other applications. The energy in both these cases exists prior to being harvested and is merely converted into something that can be used in this way. This is considered different than burning oil because, unlike oil, neither wind nor sun will ever be depleted while there are humans living on Earth. These energies also happen to be clean in that they do not typically give off emissions, but this is not necessarily related to the fact that they are renewable.
On a conceptual level, a renewable energy resource is any resource that can be converted into energy but cannot be depleted. If someone were to efficiently convert helium into energy, for example, this would not be considered a renewable energy resource because there is a limited amount of helium in the world. In contrast, various forms of moving water are considered renewable energy resources because harnessing the power of the water does not take away from the water itself.
Given the nature of these resources, renewable energy is often more cost effective than energy that cannot be renewed. The devices that harvest the energy may be expensive, but the resource from which the energy comes is often free and naturally existing. Most of the expense of renewable energy is in building and maintaining the collection devices because the energy source is often free, which is the case with wind.
It is important to understand that renewable energy resources are not always environmentally friendly resources, even though those two categories typically do overlap. Dams, for example, are considered a type of renewable energy resource but may adversely affect the environment in some cases by disrupting a river's ecosystem. Human populations typically seek out renewable energy resources because these resources have the most potential for providing energy in the long term for a civilization. It is important, however, to consider the other effects that any energy resource will have on an area.