A tap is a one-way valve that, when opened, permits beer to escape but allows nothing back in. Of course, a certain amount of pressure is lost with the exiting beer, and this is why the keg has to be re-pressurized with either air (via a party pump) or one of those fancy gases. But if you're using CO2 or nitrogen to pressurize your keg, not just any old tap will do. You need a coupler.
Couplers are taps designed for kegs that are pressurized with gas, and because different beers need different levels of pressure, there are seven different kinds of taps. American beers follow the D system, German beers follow the A system, Bass and Anchor Steam follow the G system, and so forth [source: Micromatic]. Different beers require different amounts of pressure because of differences in aroma and flavor. The more aromatic and flavorful beers are less carbonated so they need less pressure to maintain that low carbonation level [source: Wayland Works].
Then there's the matter of temperature. The warmer a beer gets, the faster it goes flat. That's because heat causes the carbon bubbles to expand and pop out of the beer more rapidly. This is especially bad news for highly carbonated lagers like Coors or Molson. In bars where the kegs are kept far from the taps, the beer must travel through long lines to the tap. To keep the beer from warming up as it travels, a glycol system keeps the lines safely insulated and cooled [source: Wayland Works].