Kegs are cylindrical beer storage vessels that are typically constructed out of stainless steel or aluminum. They come in a variety of sizes from 2 gallons all the way up to a full size 1/2 barrel keg at 15.5 gallons.
I like to consider a keg to be a home brewer’s best friend. The primary benefit of a keg over bottles is the convenience. There is only one container to clean, sanitize, fill and carbonate; kegs are also very durable and allow you to modify your carbonation level if desired.
Most home brewers use a version of a 5 gallon keg known as a corny keg or Cornelius keg. Below is a photo of three varieties of 5 gallon kegs. On the left is a 5 gallon ball lock Cornelius keg, in the center is a 1/6th barrel keg (that you would typically receive from a large scale or craft brewery) and to the right is a pin lock conversion keg; all hold approximately 5 gallons of beer.
The ball lock kegs tend to be most common and prized by home brewers. I personally use both ball lock and pin lock kegs that have been converted to ball lock so that they are compatible with my CO2 system. I use my pin lock conversion kegs as fermentation vessels in my temperature controlled fermentation freezer. The pin lock kegs are shorter and wider which allows me to fit them in my fermentation freezer without the need of a collar extension for the freezer.
Special air locks are available for use with the keg-style fermenters; I only use them as a secondary fermenter due to the reduced head space available for the foam created during primary fermentation.
Below are three varieties of 5 gallon kegs.