LPG is a fuel. The letters mean liquefied petroleum gas: gas that comes from oil, kept as a liquid in a strong bottle so you can carry it and store it. When you open the tap, it turns back into gas and you burn it for heat or power — a stove, a water heater, a room heater, or, in some places, a car or a bus. It is mostly propane and butane, not the same stuff as the gas that comes through pipes under the street (that is usually natural gas, mostly methane). A smell is added on purpose, because the real gas has almost no smell and you need to notice a leak. It is handy where there are no pipes; the bottle is under pressure, so people treat it with care. Though actually, in everyday talk people often just say “gas” and do not bother with the letters.