Indonesia is the world's largest exporter of thermal coal. Indonesia surpassed Australia for that 'title' in 2009 and has since moved further ahead as the leading exporter of thermal coal.
Thermal coal is coal used primarily for power production, where it's prime use is to produce steam to drive steam turbines for electricity generation. Thermal coal is differentiated from Metallurgical or coking coal which is primarily used in blast furnaces for steel production.