The concept of "natural resource" is particularly broad and intuitively clear to most people. It could be defined, according to the Merriam Webster dictionary as "something (as a mineral, waterpower source, forest, or kind of animal) that is found in nature and is valuable to humans (as in providing a source of energy, recreation, or scenic beauty". However, it appears far more difficulto to set clear cut boundaries to what is to include in such a definition.

 Natural resources can be classified as follows:

• Non renewable and non recyclable resources such as fossil fuels
• Non renewable but recyclable resources, such as minerals
• Quickly renewable resources such as fish
• Slowly renewable resources such as forests
• Environmental resources, such as air, water and soil
• Flow resources, such as solar and wind energy

The issue of depletion plays an important role in the use of non renewable and renewable natural resources. In the renewable resources depletion occurs when extraction exceeds renewal rate. Environmental services include the sink function which assimilates and recycles waste products from production and consumption. Flow and environmental resources are not depleted and always exist.

However, environmental resources can be degraded by pollution, and rendered useless.

(Source: Sustainable Resource Use and Sustainable Development: A Contradiction?)