Explanation:

The four types of coal are peat, lignite, bituminous, and anthracite. Peat is often not listed as a type of coal since the use of it as an energy source is limited today. However, it's still a type of coal and can be used as an energy source.

Peat is the first step in coal formation. Peat is composed of over 60% organic matter; typically, ferns and vegetation found in swamps or bogs. As a result of the high water content of this environment, peat contains a lot of water, which limits its heat content or the amount of energy it contains. It's a very soft brown coal.

Eventually over time, with increasing pressures and temperatures, peat is 'cooked' into coal's next stage, lignite. Lignite is a soft brown coal that still contains a high amount of water. Lignite has a higher heat content than peat but is still not the most desired form of coal. However, lignite makes up almost half of our known coal reserves.

Bituminous coal is formed as more pressure is applied to lignite coal. The greater the pressure applied, the more water is expelled, which increases the amount of pure carbon present and increases the heat content of the coal. Bituminous coal is often classified as sub-bituminous or bituminous. The difference is that sub-bituminous is the transition stage from lignite to bituminous coal. Bituminous coal is widely used in the United States and across Europe.

Anthracite coal is a metamorphic rock and is considered the highest grade coal. It's hard and dark black in color. It has a very light weight when compared to other forms of coal, as there is very little water present in anthracite. As a result, anthracite has the highest heat content. Anthracite is formed when bituminous coal is subjected to great pressures, such as those associated with the folding of rock during the creation of mountain ranges.