Both the terms occurrence and reserve refer to some amount of a resource, however, the ideas are fundamentally different. Primary fuels such as coal, oil, natural gas, and uranium are materials that occur on the Earth, and have associated occurrences and reserves. However, flows in nature like wind, solar and hydro only have associated occurrences, and no reserves as there is no "stockpile" of them. Generally, the term reserve usually refers to fossil fuel reserves.
The term occurrence refers to any presence of a resource that exists. The total occurrence is all of the resource in nature. An energy flow's total occurrence is any instance the flow is able to be used at any given moment. The total wind occurrence is all of the wind that's blowing, whether it's useful or not. For fuels, the total occurrence includes the discovered and undiscovered deposits, economically recoverable or not economically recoverable. For coal, total occurrence refers to every deposit of coal in the world, regardless of whether or not it will ever be useful.
On the other hand, reserves are deposits of fossil fuels that are known to exist with a reasonable level of certainty based on geological and engineering studies. These reserves are also recoverable economically with the technologies that already exist. This means that the amount of fossil fuel in a reserve will always be less than the total occurrence of the fossil fuel. Essentially, reserves represent the current amount of fuel that can be extracted in the near future.
Since occurrences include undiscovered fuel deposits, there is a varying level of uncertainty on the quantity of a resource, while the size of reserves can be estimated to a fairly certain degree. This does not mean that there is no way to estimate the size of these occurrences, the total amount of a resource can be described and estimated based on information obtained from surveys.
The difference between an occurrence and a reserve can be explored using a McKelvey diagram. A simple adaption of the McKelvey box is seen in Figure 1, and it portrays information about how a resource can be classified. The entire box represents the resource's occurrence and within it are various categories. Moving from left to right means the certainty of existence for an occurrence decreases. Moving from the top to bottom represents the decrease in economic feasibility (usually when prices for a a fuel goes down). The bottom left corner (conditional resources) represents resources that aren't economically feasible to extract, but are known to exist to a fairly high certainty. The right side (hypothetical/undiscovered resources) represent ones that aren't known to exist (and therefore can't be economically feasible!) Lastly, the top, left corner represents the reserve. A true Mckelvey box will go more in depth on the varying certainties of a reserve (known as the 3 P's of reserves), which can be explored on its main page, here.
It's important to recognize that when reserves go up (and they do! See the reserve page) that's because of improved technology, higher prices (better for the economy) or newly found deposits are making a portion of the resource's total occurrence possible and economical to mine.