A resource can be renewable or non-renewable. Renewable resources are those resources which can be regenerated, whereas non-renewable resources are those resources which cannot be regenerated once they are exhausted. Fossil fuels are non-renewable resources as a finite quantity of coal, oil and natural gas is present on our planet which may be consumed completely.

Their forma­tion requires millions of years which cannot occur within the human scale of time. Similarly, metals and minerals come from deposits developed by a very slow process of geo-chemical concentration which took millions of years to form. Though the mineral elements are inexhaustible, i.e., we cannot consume them irrecoverably, yet the concentrated deposits which occur today can disappear at some point of time in future.

Wood, fiber, fodder, fruits, vegetables, meat, milk and milk products etc. resources which are developed directly or indirectly by recent photosynthetic activity are renewable resources. They can be generated again and again, as long as photosynthesis continues on this planet. However, it does not mean that these resources are unlimited. Consumption faster than their regeneration not only causes their depletion but also tends to damage the very system which is responsible for their production.