Liquefied petroleum gas (LPG) is basically the same type of gas that is used in home barbeques. For automotive applications, it is a mixture of propane gas and butane gas.
The LPG mixture is kept under heavy pressure in a special tank in the car, so it is liquid there. LPG assures a very clean combustion. That’s because before entering the engine, LPG is converted from liquid to gas, so it can be perfectly mixed with the air, unlike gasoline. The exhaust emissions of engines running on LPG are very low.