The term resource refers to the total amount of a material or primary energy flow that exists in nature. To emphasize where these resources come from they're often referred to as natural resources. This includes material that is both discovered and undiscovered, economically recoverable or not economically recoverable. While this encyclopedia focuses mostly on 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 material resources. Flows in nature like wind, and hydro are also natural resources. Since resources include undiscovered fuel deposits, there is a certain level of uncertainty of how much of a resource there actually is. This does not mean that there is no way to estimate the size of these natural resources, the total amount of a resource can be described and estimated 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, although it is difficult to "quantify" so generally resources are used to refer to fossil fuels.
The fact that coal, oil, and natural gas are referred to as non-renewable resources can be confusing, as it makes it sound as if these resources are running out soon. It is true that fossil fuel reserves are predicted to decline in the future, (see Hubbert's peak for some description of this). However vast resources will still remain simply because there is likely so much undiscovered or inaccessible fuel.
Although both resources and reserves refer to some amount of a mineral or fuel, they are not the same thing and should not be confused as they are very different. This difference can be explored using a McKelvey diagram. McKelvey diagrams portray information about how a resource can turn into a reserve over time with effort.
Resources contain a large amount of energy sources, so it is sometimes easier to divide the resource as a whole into smaller pieces to understand why it is not a reserve. The types of resources that exist are:
Combined, the identified and undiscovered amounts for a resource are the total resource. Although some resources may be currently undiscovered or inaccessible, it is possible in the future that further exploration and advances in technologies could allow these resource to become known and economically viable. If this occurs, the worlds reserves will become larger.