Compressed natural gas, or CNG, is stored at high pressures in order to maintain its usability, whereas liquefied natural gas, or LNG, is natural gas cooled down below its boiling point to separate it from any components that might make it less efficient or more dirty. Both of these are different ways of storing methane gas mixtures composed of virtually the same type of chemicals.
Compressed natural gas is stored at pressures that compress the gas down to less than one percent of its natural volume, forcing it into liquid form. CNG has been used for as far back as World War II, with developments made to increase its efficiency since then.
Liquefied natural gas does not require the massive pressure that CNG does because it remains in a liquid state as long as it is kept cold. LNG is warmed carefully by heat exchangers to convert it back into its gaseous form when needed, and it has to be stored in special tanks insulated for the low temperatures. Due to the constant slow heating of LNG even when stored properly, these tanks need to be properly vented in order to prevent ruptures. This vented gas is used as fuel by the facilities and vehicles that store and transport it.