Coal is a fossil fuel formed in swamp ecosystems where plant deposits were saved by water or mud from oxidization and biodegradation. Coal is a sedimentary, organic rock, which is mainly composed of carbon, hydrogen, oxygen, nitrogen and varying amounts of sulphur.

Coal is extracted from the ground by mining, either underground or open pit mining.

According to the geological characteristics, dead matters are successively transformed into the different types of coal:

• Peat is considered to be a precursor of coal.
• Lignite, also referred to as brown coal, is the lowest rank of coal and mostly used for power generation.
• Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal is used primarily for power generation.
• Bituminous coal, a dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke.
• Anthracite, the highest rank; a harder, glossy, black coal used primarily for the heating of residential and commercial buildings.

The classification of coal is generally based on the content of volatiles. However, the exact classification varies between countries.