A beer tap works by using air pressure to force the beer out of the keg, through the tube and into the glass, and the air pressure can either come from CO2 tanks or a manual pump. Understand how beer taps work with information from a professional bartender in this free video on beer.
How does a beer tap work? At a bar, such as this, you will have a tap that has two lines in it. One of them is a line that brings the beer out to here. And one of them is a line that leads to a CO2 tank. And the CO2 tank just uses air pressure to force the beer out of the keg and through the lines, ultimately to here where you can pour it into the glass. And, of course, you know your CO2 will eventually run out. You'll need to change your CO2 tank, just like your beer will run out of your keg, and you'll need to change that. If you are at a party or you get a keg on your own, you will have a different type of tap, and that's one that will have a pump on it. So you will manually have to pump the tap to push air into the keg and force the beer out manually. And that will just have a small tube with a little pourer on the end. The two are basically the exact same concept though. But for obvious reasons at a bar you need to have the CO2 tanks, because you have multiple kegs, long lines and you can't be pumping beer on a Friday night manually when you got four deep at the bar.