Brewing

Beer is made by brewing. The essential stages of brewing are mashing, sparging, boiling, fermentation, and packaging. Most of these stages can be accomplished in several different ways, but the purpose of each stage is the same regardless of the method used to achieve it.

A 16th century brewery
Mashing manipulates the temperature of a mixture of water and a starch source (known as mash) in order to convert starches to fermentable sugars. The mash goes through one or more stages of being raised to a desired temperature and left at the temperature for a period of time. During each of these stages, enzymes (alpha and beta amylase primarily) break down the long dextrins that are present in the mash into simpler fermentable sugars, such as glucose. The number of stages required in mashing depends on the starch source used to produce the beer. Most malted barley used today requires only a single stage.
Sparging (a.k.a. lautering) extracts the fermentable liquid, known as wort, from the mash. During sparging the mash is in a vessel known as a lauter-tun, which has a porous barrier through which wort but not grain can pass. The brewer allows the wort to flow past the porous barrier and collects the wort. The brewer also adds water to the lauter-tun and lets it flow through the mash and collects it as well. This rinses fermentable liquid from the grain in the mash and allows the brewer to gather as much of the fermentable liquid from the mash as possible. The leftover grain is not usually further used in making the beer. However in some places second or even third mashes would be performed with the not quite spent grains. Each run would produce a weaker wort and thus a weaker beer.
Boiling sterilizes the wort and increases the concentration of sugar in the wort. The wort collected from sparging is put in a kettle and boiled, usually for about one hour. During boiling, water in the wort evaporates, but the sugars and other components of the wort remain; this allows more efficient use of the starch sources in the beer. Boiling also destroys any remaining enzymes left over from the mashing stage as well as coagulating proteins passing into the wort, especially from malted barley, which could otherwise cause protein 'hazes' in the finished beer. Hops are added during boiling in order to extract bitterness, flavour and aroma from them. Hops may be added at more than one point during the boil. As hops are boiled longer, they contribute more bitterness but less hop flavour and aroma to the beer.
Fermentation uses yeast to turn the sugars in wort to alcohol and carbon dioxide. During fermentation, the wort becomes beer. Once the boiled wort is cooled and in a fermenter, yeast is propagated in the wort and it is left to ferment, which requires a week to months depending on the type of yeast and strength of the beer. In addition to producing alcohol, fine particulate matter suspended in the wort settles during fermentation. Once fermentation is complete, the yeast also settles, leaving the beer clear. Fermentation is sometimes carried out in two stages, primary and secondary. Once most of the alcohol has been produced during primary fermentation, the beer is transferred to a new vessel and allowed a period of secondary fermentation. Secondary fermentation is used when the beer requires long storage before packaging or greater clarity.
Packaging, the fifth and final stage of the brewing process, prepares the beer for distribution and consumption. During packaging, beer is put into the vessel from which it will be served: a keg, cask, can or bottle. Beer is carbonated in its package, either by forcing carbon dioxide into the beer or by "natural carbonation." Naturally carbonated beers may have a small amount of fresh wort/sugar and/or yeast added to them during packaging. This causes a short period of fermentation which produces carbon dioxide.