To back up to the very beginning, fossil fuels are non-renewable energy sources formed over hundreds of millions of years as prehistoric plants and animals decayed under layers of sand, clay, and rock. The resulting fuel - coal, oil, or natural gas - was determined by the type of organic matter, the amount of pressure exerted across time, and at what temperature.
Reserves are areas where the existence of fossil fuels is "proven, provable, or possible" and where their extraction from the ground is technologically and economically feasible.1 This is a dynamic concept since today's technologies enable companies to pull resources from increasingly complicated locations at decreasing costs, expanding the number and type of reserves across continents.
Conventional oil and gas reserves tend to be the first targets of the industry since their extraction requires only standard equipment or processes. High-impact or unconventional fossil fuel reserves include thermal coal, oil sands, shale oil, and shale gas - those fuels that require more effort to extract from the ground.