The term resource refers to any quality or substance that is valued by humans because it has the ability to meet a need or desire. The resources that the Energy sector is usually concerned about are natural resources. While this encyclopedia focuses mostly on flow resources and fuel resources, natural resources include other raw materials that come from nature like copper or aluminum in the form of ores.
From an energy perspective, primary fuels such as coal, oil, natural gas, and uranium are natural resources. Moreover, flows in nature like wind, and hydro are also natural resources. Unlike flows, primary fuels are in deposits within the Earth that must be discovered and extracted. Many deposits of these natural resources have been discovered, but many deposits have yet to be discovered. The term occurrence is used to describe all the natural resources that exist on earth—undiscovered and discovered. The oil and gas industry refers to this as "total petroleum in place". That industry also uses the term "resource base" usually means the deposits that are assumed to someday be part of the reserves.
Reserves are the natural resources that have been discovered; can be accessed with current technology and economically feasible to extract (it's profitable). It is important to note that energy flows, like wind do not have associated reserves, since there is no depository of them that has yet to be discovered. only being used when available. Estimations of the total occurrence of natural resources are based on information obtained from surveys.
Figure 1. Oil resources are fairly expansive and are one source of energy.
Figure 2. Wind is a resource which is not consistently available, which decreases its reliability. 
Coal, oil, and natural gas are referred to as non-renewable resources but this can be confusing, as it makes it sound as if these resources are running out soon. Fossil fuel reserves are predicted to decline in the future (see Hubbert's peak for some description of this), however, this is not likely to be what limits fossil fuel use. Instead, the need to slow climate change will most likely drive cause an eventual decline in fossil fuel use. A vast amount of these natural resources will remain in place because there is a limit to how much carbon dioxide can safely be put in the atmosphere. This means even though fossil fuels are non-renewable, much will remain in the ground because so much is undiscovered or inaccessible now, and will be used less in the future.
Both resource occurrence and reserves refer to some amount of a mineral or fuel in the ground, but they are not the same thing. This difference can be explored using a McKelvey diagram. McKelvey diagrams portray information about how an occurrence of a natural resource can turn into a reserve over time.
Natural resources exist in a variety of deposits. These deposits come in a range difficulties to extract, and there is a range of confidence in their existence. It's helpful to divide the occurrence as a whole into smaller pieces to understand why specific deposits might not be a reserve. The types of resources that exist are:
Combined, the identified and undiscovered deposits are the total occurrence of a natural resource. Some deposits are currently undiscovered or inaccessible, but in the future further exploration and advances in technologies could allow these deposits to become known and economically viable. If this occurs, these deposits are added to the reserves and the world's reserves become larger.