A keg of home brew - MMMMM. On the right is a Cornelius or 'Corny' keg. Cornelius is a popular manufacturer of soda kegs. Five gallons is the most commonly found size although 2.5 and 10 gallon kegs are available at a premium.
On the left is a CO2 cylinder. Attached is a regulator with high and low pressure gauges. Anti-backup valves are connected between the regulator output and the gas feed lines. This insures that if one keg has higher pressure than the other it won't backup into the lower pressure keg.
The kegs have two different styles of connections. Pepsi® uses ball-lock and Coke® uses pin-lock. Also the dimensions on the 'In' and 'Out' are slightly different to keep you from accidentally connecting them backwards.