CNG =
https://en.wikipedia.org/wiki/Compressed_natural_gas
LPG (Propane) =
https://en.wikipedia.org/wiki/Liquefied_petroleum_gas
one is compressed gas, the other is liquid, both are used for cars but not simultaneously
Volkswagen and a couple others sell CNG cars directly from factory, LPG is usually installed as a replacement for regular gasoline.
over here both are cheaper than gasoline and both have a better CO2 balance