Keg is both the name of a container for beer, and the process of keeping and serving beer under pressure
using carbon dioxide
Kegs are cylindrical in shape, with only one opening. When the keg is not in use, the opening is fitted with a screw cap. Even when in use, kegs are usually stored upright, with the opening uppermost.
To prepare the keg for serving, the screw cap is replaced with an assembly of valves and pipes, connected to a CO2 cylinder, and to the beer line that feeds a bar tap.
Keg beer was introduced in the UK in the 1960s and 1970s. The biggest breweries looked on keg beer as a cost saving, in that it keeps for longer, and requires less care and skill in the cellar than real ale. Watney's brewery introduced a beer called Watney's Red, launching it with a quasi-communist advertising campaign: bring on the Red Revolution. This beer was later renamed Watney's Red Barrel, which came to epitomise poor quality mass produced beer - and was even target of some ridicule in Monty Python.
1971 saw the setting up of the Campaign for Real Ale, which has become one of the most successful consumer pressure groups. Keg beer was the primary target and raison d'etre for the campaign.
CAMRA is still active in it's battle against keg. The latest adversary is Nitrokeg - an invention of the big brewers in the early 1990s, where a mixture of Nitrogen and CO2 gases is used. This produces a more frothy head on the beer, and is supposed to appeal to a younger target audience. Guinness was the first beer to be served in bulk using nitrokeg, and the big brewers are still trying to 'educate' the consumer pallettes that smoothflow and cream beer is acceptable.