Compressed natural gas (CNG) is a fossil fuel substitute for petroleum-based gasoline, diesel, or propane. It does not produce greenhouse gases and is a cleaner alternative fuel, as well as being safer (if it's spilled). CNG is made by compressing natural gas (which is mainly composed of methane to less than 1% of the volume it occupies at standard atmospheric pressure). It is stored and distributed in hard containers.
CNG can be used in traditional gasoline internal combustion engine vehicles that have been converted to bi-fuel vehicles (gasoline/CNG). It has become increasingly popular due to its less emissions and relative safety.