Propane and butane are short chain alkanes with 3 and 4 carbons respectively. They are both gases at normal temperaures. They come out of the top of the distillation tower and are separated before being bottled.
However, they can be pressurised to turn them into a liquid. We then refer to them as Liquified Petroleum Gas.
Once they have been liquified, they can be put into pressurised cylinders and sold for domestic and industrial use. Butane cylinders are used for gas barbecues and camping and caravan heaters. Propane is used as a substitute for natural gas in remote areas. So it can be used for central heating systems, cookers and so on.
The cylinders have to withstand the high pressure of the gas inside. This is why they are made from strong welded steel.