There are several different types of beer or keg taps:

Pressure-Dispense
Almost universally in modern times, bulk beer is supplied in kegs that are served with the aid of external pressure. In a normal bar dispense system, this pressure comes from a cylinder of carbon dioxide (or nitrogen) which forces the beer out of the keg and up a narrow tube to the bar. At the end of this tube is a valve built into a fixture on the bar.

Portable keg tap
Sometimes, beer kegs designed to be connected to the above system are instead used on their own, perhaps at a party or outdoor event. In this case, a self-contained portable tap is required that allows beer to be served straight from the keg. Because the keg system uses pressure to force the beer up and out of the keg, these taps must have a means of supplying it.

Cask beer tap
Beers brewed and served by traditional methods, typically cask ale, do not use artificial gas. Taps for cask beer are simple on-off valves that are hammered into the end of the cask. When beer is served directly from the cask, as at beer festivals and some pubs, it simply flows out of the tap and into the glass.