Using Cornelius kegs, also known as soda kegs or Corny kegs, is a very efficient way of storing and dispensing your home brew. These kegs can be acquired at your local brew store, and sometimes at restaurant supply stores. If kept in good condition, these kegs not only allow easy access to your brew, but also pave the way for you to have your own home brewed beer on tap at home! There are two kinds of airlocks on these kegs - ball lock kegs and pin lock kegs. If you have ball lock kegs, you will not be able to use pin lock keg attachments on it, and vice-versa. The two systems are incompatible, yet very similar. With either system, there are a few techniques (and concerns) which are usable only with a Corny keg.
The main concern using Cornelius kegs is to clean them thoroughly before and immediately after use. To do this, you will need a few special tools that will allow you to take off the air and liquid locks at the top of the keg. You should check your gaskets each time you do this, as a little too much wear will cause them to leak. This could possibly contaminate your brew! Check in with your local home brew supply store or with the company you bought your Cornelius keg from about buying extra gaskets for your keg. It is always good to have these on hand.
You will also need a carbonation (CO2) tank and/or an air pump in order to get the beer out of your ball lock keg or pin lock keg. I recommend having both in your arsenal. The CO2 tank is useful for having beer on tap all year round, preferably in a kegerator to keep it ice cold (the beer, that is, not the CO2 tank). The CO2 tank is also ideal for bringing to a small or medium sized party. If you are serving home brew at a large party, one where you expect to drink all the beer in the keg, I recommend the air pump. This will save you money on CO2, and the air pump is easier to carry than the CO2 tank. If you use an air pump, you have to drink all the beer by the next morning. If the beer is not finished, the remainder will most likely become flat and contaminated by air-borne bacteria. But if a CO2 tank is used, the beer will remain good no matter how many weeks it is before you empty the keg.
There are three ways to carbonate your beer in a Cornelius keg. The most common is to utilize a CO2 tank. After you put putting your beer into the ball lock keg or pin lock keg, turn your CO2 regulator up very high (around 50 psi), and roll the keg around on the ground with your foot. By shaking the beer and CO2 mixture, you force more CO2 into the mixture. It is recommended to wait one or two weeks before tapping the keg to let the CO2 have plenty of time to absorb into the beer. Here's another useful tip: by putting your keg into the cellar, refridgerator, or a kegerator, you will carbonate your beer faster! CO2 mixes with beer more easily at lower temperatures.
The second way to carbonate your beer in a keg is to approach it just like one big bottle - adding an appropriate amount of corn sugar to the keg, and leaving the yeast to do its work over the next two weeks.
The third way is natural carbonation. Judge when your beer is nearly finished fermenting, and before all the sweetness is gone, put it in the keg to pressurize. This way, the naturally occuring ingredients in your brew do the carbonating for you. But beware - too soon or too late and you will end up with a brew either too foamy with yeasty tones of flavor or a brew that is not carbonated enough for your tastes.
Now as to the difference between ball lock kegs and pin lock kegs - I cannot say that there is much. On pin lock kegs, the "gas-in" port has two pins, while the "liquid-out" has three, making it impossible to accidentally attach the hoses backward. The main thing to remember is that the two systems are not interchangeable. I prefer ball lock kegs because they are easier for my hands, but by the same token, pin lock kegs may be easier for others. It is best to try each and see which you prefer, but only go with one kind in your brew shop!