The different types of coal depends on the organic maturity of plant remains. This maturity obviously happens during the geologic ages and produced different types of coal that offer different characteristics (color, hardness, oxygen, hydrogen and sulphur balance). These different types of coal are consequently used for different purposes.
Starting from the peat, the first output of the conversion process is the lignite (also known as "brown coal"). The continuing effects of temperature and pressure might transform lignite into sub-bituminous coals. Sub-bituminous coal might become harder and blacker, being then converted into bituminous coal (also known as "hard coal"); anthracite coal being is at the end of the evolution chain, containing the lowest rate of impurities.