From Wikipedia: Draught beer (Draft beer in U.S.) is beer served from a cask or a pressurised keg.
Until Joseph Bramah patented the beer engine in 1785, beer was served directly from the cask and carried to the customer. The Old English word for carry was dragen which developed into a series of related words, including drag, draw, and draught. By extension, the word for carrying or drawing a beer came to mean the serving of the beer and, in some senses, the act of drinking, or a drink of beer itself, regardless of serving method. By the time Bramah’s beer pumps became popular, the use of the word draught to mean the act of serving beer was well established and transferred easily to beer served via the hand pumps.
In 1691, an article in the London Gazette mentioned John Lofting, who held a patent for a fire engine: “The said patentee has also projected a very useful engine for starting of beer, and other liquors which will draw from 20 to 30 barrels an hour, which are completely fixed with brass joints and screws at reasonable rates”.
In the early 20th century, serving draught beer from pressurised containers began. Artificial carbonation was introduced in the United Kingdom in 1936, with Watney’s experimental pasteurised beer Red Barrel. Though this method of serving beer did not take hold in the U.K. until the late 1950s, it did become the favoured method in the rest of Europe, where it is known by such terms as en pression. The method of serving beer under pressure then spread to the rest of the world; by the early 1970s, draught beer was almost exclusively beer served under pressure.
Shortly after a British consumer organisation called the Campaign for Real Ale (CAMRA) was founded, in 1971, to protect unpressurised beer, the group devised the term real ale to differentiate between beer served from the cask and beer served under pressure. By 2004, the term real ale had been expanded to include bottle-conditioned beer, while the term cask ale had become the accepted global term to indicate a beer not served under pressure.
Keg beer is a term for beer which is served from a pressurized keg. While often considered synonymous to draught beer, keg beer refers specifically to beer served under pressure, while draught beer may refer to any beer served from a larger container, including both keg beer and cask ale. Keg beer is often filtered and/or pasteurized, both of which are processes that render the yeast inactive, increasing the shelf life of the product.
In brewing parlance, a keg is different from a cask. A cask has a tap hole near the edge of the top, and a spile hole on the side used for conditioning the unfiltered and unpasteurised beer. A keg has a single opening in the center of the top to which a flow pipe is attached. Kegs are artificially pressurised after fermentation with carbon dioxide or a mixture of carbon dioxide and nitrogen gas.
Keg has become a term of contempt used by some, particularly in Britain, since the 1960s when pasteurised draught beers started replacing traditional cask beers. Chemicals (adjuncts) such as glycerol monostearate, are used to create a foam or head on the finished beer.