Beer kegs are typically made out of stainless steel, but you may occasionally run into one that is made out of aluminum. A beer keg is a cylindrical container and is available in various sizes. The keg sizes are not standardized around the world, so a 1/2 keg in the United States will not necessarily be the same amount of beer as a 1/2 keg in the United Kingdom.
Beer kegs have a single opening on one end and are designed to be used with the opening on the top. There is a tube that extends from the opening to the other end of the keg which is called a “spear”. A self closing valve is located at the top of the spear. Also located near the top of the spear is an opening which is used to allow gas (usually carbon dioxide) into the keg to force the beer out of the keg.
When you “tap the keg” by attaching one of two types of taps to the top of the keg, you are forcing the self closing valve to open, which will allow the beer to flow through the tap given their is pressure from either regular air as is the case with the type of tap that you have to pump to get the beer to flow, or carbon dioxide if you use the tap that allows for carbon dioxide to be used to pressurize the inside of the keg.
Using the type of tap that you must pump to pressurize the keg will introduce bacteria into the keg and cause the beer to go bad more quickly. This tap should only be used if the beer is going to be consumed within approximately 18 hours. Any longer and the beer may become unpalatable (taste skunked). If you plan on making the beer in the keg last any longer, you should use carbon dioxide to pressurize the inside of the keg. Kegerators use carbon dioxide which will reduce the outside contaminates and bacterias from entering the keg and spoiling the beer.
Whether you are hand pumping air or using carbon dioxide, the pressure builds up inside the keg and when the lever is flipped to open the spout on the tap, beer is forced through the spear, past the self closing valve which was opened when the keg was tapped, and out of the keg into your cup. Too easy.
Never attempt to cut open or pierce a keg unless you are 100% sure that there is no pressure inside the keg. Even after the beer is gone, pressure may remain and you could get seriously injured or killed.