Above is a basic drinks dispense diagram showing how to install a single point of draught/keg beer on a typical UK set-up.
GAS - The gas bottle runs to the primary valve, the secondary valve and then into the side of the keg connection.
BEER- The beer/lager runs from the keg connection to the cellar buoy, to the inlet of the flash cooler. Then from the flash cooler into the beer tap. There is usually a flow controller between the outlet of the cooler and beer tap to adjust how fast the beer is poured.
How to install draft beer (draught)