Question: Do all the different types of kegs have the same type of keg tapping equipment?
Answer: No. If you are a home brewer, many of the kegs you will run into will be Cornelius kegs, or "Corny" kegs, which have been originally used for soda fountains. These days, soda fountains use plastic bags instead, so Cornelius kegs are becoming more and more rare. Hold on to them if you can get a hold of them! In the US, there are seven different beer keg systems in use. These are as follows:
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"D" System (American Sankey): most  domestic beers i.e. Budweiser, Coors, and Miller
                  "S" System (European Sankey): most  import beers i.e. Heineken, Becks, and Amstel Light
                  "A" System (German Slider): many  German beers i.e. Spaten, Warsteiner, and Paulane
                  "G" System: a wide variety of beers  i.e. Anchor Steam, Bass, and Caffrey'
                  "U" System: many English beers i.e.  Guinness, Harp, and Boddington
                    Twin Probe: limited use by some  microbreweries
                    Home Brew Coupler: ball lock  fittings used for home brew tanks
Question: What are the most common keg troubleshooting questions?
Answer: If your beer is soured or cloudy, cleaning your beer lines is the first thing you should do. If you have used the toughest beer line cleaner available and are still having problems, it may be time to take your lines and keg apart completely and sanitize all the individual little parts separately and then flush the whole system one more time before going back to beer drinking. If you are a home brewer, please ensure that the problem does not stem from old sediment in the bottom of the keg or up spout, improper ageing, or bacterial contamination from the carboy.
If your beer has improper carbonation, a common issue, you should make sure that the beer is at the right temperature. It can take over 10 hours for a keg to be chilled to the regulation 36-38 degrees Fahrenheit required to keep the beer in top drinking condition. Don't jump the gun on this, or your beer may be wasted - 25% of the volume of the keg may be lost due to too much foam.
If you are lacking pressure, make sure your CO2 tank is on, and the pressure is between 12-14 Lbs.
WILD BEER: Beer, when  drawn, is all foam, or too much foam and not enough liquid beer.
                    Either the  beer is too warm (check your kegerator's thermostat) or you may have either a  blockage in your beer line or a maladjusted CO2 tank (should be between  12-14lbs.). See Above
FLAT BEER: Foamy head  disappears quickly; beer lacks brewery fresh flavor.
                    It looks like your beer is maybe TOO  cold? "Coldest Beer in Town" isn't  always what you are looking for. The  beer might be getting stale if it SOMEHOW has been sitting around not being  drunk for over 4 months after carbonation is complete. If home brewed, were the ingredients fresh?
CLOUDY BEER: Beer in  glass appears hazy, not clear.
                    Check that thermostat again. You might have frozen or nearly frozen your  beer. I once found an unopened bottle of  MGD that looked like this after sitting in a graveyard un-refrigerated for  years. A word of advice: if the keg has  been sitting around in your yard for that long, it's not worth contaminating  your beer lines to try it out.
FALSE HEAD: Large soap-like bubbles, head dissolves very  quickly.
  Amateur  bartender? Might be a beer line blockage  as well. Maybe the CO2 is real low AND  the beer was not properly carbonated.
Question: What are the specifications for common keg types?
Answer:
                    Home Brew: Bev.Keg.(Cornelius keg):
  5 gal., 640oz,  53x12oz glasses, 25"tall, 8.5"dia, 49 lbs full
                    
                    Sixth (1/6th) barrel
                    5.23 gal, 669oz,  55x12oz glasses, 23.3"tall, 9.25"dia, 56 lbs full
                    
                    Short Quarter (1/4) barrel aka  "Pony Keg"
                    7.75 gal, 992oz,  82x12oz glasses, 14.8"tall, 17"dia, 81 lbs full
                    
                    Slim, ( Tall ) Quarter (1/4) barrel
                    7.75 gal, 992oz,  82x12oz glasses, 23.3"tall, 11"dia, 81 lbs full
                    
                    Half (1/2) barrel
                    15.5gal, 1984oz,  165x12oz glasses, 23.3"tall, 17"dia, 161 lbs full
Question: How long will a keg of beer stay fresh?
Answer: If you are using a well-maintained kegerator, beer will keep for about 4 months, up to six months if you disconnect the beer and gas lines during that time. If you lower the temperature to about 30 degrees, it can keep for longer, but why not just buy some fresh beer instead of storing it in your kegerator? If you are a home brewer, or have bought an un-pasteurized keg of beer, the beer will definitely undergo changes in flavor during this time, but it won't necessarily go bad.
If dispensing with an air pump, I can guarantee no more than 8 hours, but it might still be drinkable after a couple of days if you are lucky.
Question: What is the optimal pressure setting for the CO2 regulator?
Answer: 12-14 PSI will do nicely for most  beers. Remember that temperature is as  vital to getting the proper head as the CO2 regulator setting. To keep your keg of beer tasting fresh, keep  the regulator at 12-14 PSI. For more  carbonation, go up to 15-16 PSI, but turn the temperature of the beer way down  to 32 degrees.
           
Drink it in  good health!