Beer

Ale
Ale is a top-fermented beer that originated in England, as early as the 7th century. The top fermenting yeast, Saccharomyces cerevisiae, performs fermentation at a higher temperature of between 55 °F and 70 °F (13-21 °C) than the yeast used to produce lager, and their byproducts are more evident in the final taste and aroma of the ale.

Ale is properly supplied to the point of sale as a "living" beer in which the yeast fermentation continues to occur. Managing the final stages of this process in the cellar or special cool-room is a skilled task, but this is the role of the cellarman rather than the barman or barmaid and is not considered further here.

Ale can be served in two ways - from a beer engine or by gravity. The latter is the simpler technique, in which the glass is filled directly from the cask which is usually placed on a counter behind the bar. The tap in the end of the cask may be an ordinary threaded beer-line tap with a twisting action, or it may be a pouring tap with a lever-style handle. The glass is simply placed close under the tap and filled. Gravity service is usually found in ale-centric establishments with many beers on at once, and at beer festivals.

In most British pubs, the beer engine is used instead. This is the proper name for the tall, manually operated hand pump on the top of the bar, which is connected via the beer-line to the cask in the cellar. Most pumps dispense around a half-pint per pull, although they are frequently worn and require several pulls to obtain a complete pint. Quarter-pint engines also exist. The handle should be operated in a smooth manner across the whole of its arc, rather than jerked rapidly to and fro which tends to agitate the beer, creating a large head into which much of the bitterness migrates.
Head size is also regulated by the sparkler which may be fitted to the outlet of the pump. This forces the beer through small holes, deliberately agitating it and producing a large head. Some beers are brewed with this in mind, and others are not - a good barman will use a sparkler only on those beers meant to be served through one. In the UK, CAMRA's Good Beer Guide indicates whether this is the case; most brewers will also tell their customers, and may even affix labels with these kind of instructions to the casks.

Many pumps are fitted with a "swan neck" designed to reach to the bottom of the glass. If a sparkler is fitted it is important to use this in order not to create an excessive head which must either be allowed to drop in its own time or spilled out of the glass and wasted. The glass should be raised around the swan-neck so that the outlet is at the bottom. It should be left there until the glass is almost full.

Stout
Stout is an ale in itself. It is a dark beer, sometimes black in colour. It is made with roasted grain.
The most famous variety of stout is Guinness, originally from Ireland but now brewed in various countries around the world. Guinness has a light body, with a distinct astringency, and is known for its creamy head of Nitrogen foam.
Scottish stout is less bitter and in the fruity side.
Sometime oat is added during the making of Stout to make the Oatmeal variety. Traditionally, the matching food for Stout is oyster. Like all other beers, Stout is usually served cool. But some drinkers around the world prefer having Stout in room termperature.
Stout was originally known as "Stout Porter" to emphasize its fuller body. Today, that isn't usually true. Stouts are typically distinguished by use of roasted barley, which gives it the characteristic sharp roasty taste. Porters typically achieve their color with chocolate malt, so called for its color and vaguely chocolate-like taste.

Lager
A lager is a type of beer made with specific strains of yeast which ferment at a significantly lower temperature than ale yeast, resulting in a usually crisper taste. A popular type of lager is a pilsner which is widely consumed in North America, Budweiser and Miller both being of this general style. Like Ales, Lagers also have a number of different styles of brewing.

Source: wikibooks.org - the open-content textbooks collection