An LP gas cylinder works by storing gas under pressure. To reduce the high pressure to a usable level, the gas is passed through a gas regulator to decrease the pressure to the correct level for the gas appliances. LPG is unique, as it liquefies under moderate pressure vs high pressure compressed gas cylinders.
LPG is supplied in gas bottles that are either exchanged or refilled on site by LPG tankers. When used for a BBQ, it can be refilled or exchanged for a full one in a swap scheme. These are typically 9kg bottles. SWAP'n'GO BBQ gas bottles are a good example of a swap system.