Natural resources are comodities which are considered valuable in their relatively un-modified (natural) form. A commodity is generally considered a natural resource when the primary activities associated with it are extraction and purification, as opposed to creation. Thus Minerals (mining), Oil, fishing and forestry are generally considered natural resources, while farm foods are not.
Natural resources are often split into renewable and non-renewable resources. Renewable resources are generally living resources (e.g. fish and forestry) which appear to restock (renew) themselves at somewhere near the rate at which they are extracted.
Mineral resources are generally static and, once a site's non-renewable resource is exausted, it is considered to be useless for future extraction -- barring technological improvements that allow economic extraction from the tailings.
Both extraction of the basic resource and refining it into a more pure, and directly usable form, (e.g. metal, refined oils) are generally considered natural resource activities, even though the later may not necessarily occur near the former.