The term draught beer has come to mean several different preparation methods for beer. The term can be a source of confusion because it applies to so many different processes and products. Historically, a draught beer was one that came from a cask or large container, but today it can also refer to beer prepared and served from a keg. In other cases, the term can even refer to a bottled beer, but that application is generally rejected by historians.
The word draught is an old English term that generally means "to carry," as beers were then carried from the cask directly to the customer. Since then, the term has evolved, and draught came to mean a way of preparing the beer as well as a way of drinking it: one could take a draught of draught beer, for example. Since then, the term has been applied to a few different methods of preparation, each with its supporters and detractors. The most historically accurate definition of the term, however, refers to beer served directly from a cask with no pressurization and active yeast.
When brewers began putting beer in pressurized containers, the final product became known as draught beer. In the early 20th century, when this method became common, customers did not immediately respond positively to the beer because flavor was lost so that shelf life could be extended. Purists regarded true draught beer as beer that came from a cask, not from a pressurized container. Keg beer was the result of this disagreement, and again, many people rejected keg beer because the yeast in the beer was rendered inactive during the serving process, sacrificing flavor. Keg beer uses CO2 to pressurize the beer and give it carbonation that would occur naturally, had the beer come from a cask.
More recently, the term draught beer — or its alternate spelling, draft beer — has come to refer to a specific style of beer that can be presented in a bottle or can. The taste of these canned or bottled beers is supposed to resemble that of keg beer, and it is labeled as such even though it did not come directly from a keg. Some bottled beers even come with nitrogen inserts that cause carbonation when the bottle is opened. This results in a smooth taste and a thick head on the beer, somewhat mimicking the keg beer process. Most historians and purists reject such labeling on bottles and cans and consider it to be misleading.