Cask ale is the term given to unfiltered and unpasteurised beer which is conditioned and served from a cask without additional pressure. The expression cask conditioned is sometimes also used, as is the term Real Ale.
The term Real Ale was coined by CAMRA in the 1970s to draw media attention to British cask ale and to differentiate it from draught or keg beer.
Brewing starts with water. Some of the variation between the beers brewed around the country comes from the fact that the water used has naturally occurring variations of minerals depending upon the water source. Some brewers choose to condition the water (especially if their water is 'soft') by adding salts such as gypsum or magnesium. The brewing process refers to its water as "liquor".
The next ingredient is "malt" — barley which has been allowed to germinate and then roasted. Depending on the amount of roasting, this "malted barley" will strongly influence the colour and character of the beer. The malt is ground into a coarse powder (known as "grist") which, along with the heated liquor (water) is poured into a vat called a "mash tun" for a process known as "mashing". During this process, natural enzymes within the malt convert some of the starch into sugars which then play a vital part in the fermentation process.
After mashing, the liquid (now known as "wort") is moved into a large tank known as a "copper" where it is boiled up with "hops". These add flavour, aroma and bitterness to the brew. At the end of the boil, the hopped wort is clarified in a vessel called a "hop back" and the clarified wort is then cooled.
The wort is then moved into a "fermentation vat" where a specialist yeast is then blended or "pitched" with it. The yeast converts the sugars from the malt into alcohol and carbon dioxide. After a week or so, the fresh (or "green") beer is run off into conditioning tanks for a few days.
This is the point at which the production of "real ale" (aka "cask ale") and the mass-produced beers differ. The mass-produced beers would now be pasteurised, sterilized and filtered to ensure that the brewing process had halted. The beer would then be put into steel kegs, transported to its eventual destination and served using bottled gas to force the beer from the keg out through the dispensers.
Real ale, on the other hand, is racked up into "casks" (which may be wood, but are almost universally steel these days). Dry hops may be added now as a preservative and for extra flavour. Additional sugar may be added to encourage a second fermentation. A substance called finings may be added to help the beer clear in the pub cellar.
It is then transported to its eventual destination. The beer, however, is still a "living product" at this stage and is continuing to develop and condition within the cask (hence the phrase "cask-conditioned" as a synonym for "real ale"). The beer would typically be stored in the cellars of the pub for a number of weeks until the landlord judges it ready for sale.
It should then be served without the aid of gas pressure, sometimes directly from the cask (by "gravity"), typically through a bar-mounted hand pump or "beer engine", or very occasionally through an electric pump.
Casks have two holes in them which are sealed for transport and storage. The larger bottom hole is where a tap is fitted which connects the cask to the pump. The smaller top hole is punctured with a small wooden peg (known as a "spile") a few days before the beer is due to be served. This lets air into the cask and from this point it has shelf-life of just a few of days; thereafter the beer goes off (acquires odd flavors). It is essential therefore that pubs serving real ale have sufficient turnover.
To summarise, malt is made from roasting sprouted barley and the amount of roasting influences the character and colour of the beer. The brewing process allows the sugar from the malt to ferment with the yeast producing alcohol. The hops provide the bitterness associated with the British style of beer and also add to its character. The beer is racked into casks where finings, more hops and priming sugar may be added and a 'secondary' fermentation takes place; a process essential for the best quality beers. Another name for real ale is 'cask-conditioned' beer.