LPG, or 'Liquefied Petroleum Gas' is the term we use for a group of gas liquids (light hydrocarbons).
Members of this group are mainly: Propane and Butane. There are other members like ethane but they are for specialized use only.
All LPG type substances have a common trait: They're gaseous in their normal state, but become liquids when pressurized. This is what makes the stuff so practical. It's gas you can carry around without having to deal with the major volume problem gas presents.
To transport it in gaseous form, you would actually need a tank that's 270 times bigger than the tank for the liquid gas. So, if you take a car that has a 50 litre tank and fill it with LPG, you would need a 13,500 litre tank to transport the gas. That's saving room for ya...
Right now, LPG is used all over the world for cooking, as propellant for vehicles, for air-conditioning systems and so on, however usage varies significantly per region of the world.