Yeast is the most essential aspect of brewing beer. The yeast are there to convert the sugars in the wort into alcohol. The first stage of this process is called the "lag" phase, marked by the breaking of proteins into their constituent amino acids. The ferment then enters the "respiration" phase, where the yeast absorbs oxygen and reduces the pH of the wort, so that it becomes acidic and anaerobic. The yeast breaks down the glucose sugars into carbon dioxide, water, and pyruvic acid. Pyruvic acid later becomes alcohol. Yeast cannot ferment all sugars, which is why beer still has a sweet taste at the end of the ferment. The strain of yeast will impart its own flavor although malt and hops are the main flavor components. Yeast that adds little in the way of flavors are usually described as having a "clean taste". Yeast produce three metabolic by-products that affect beer taste: phenols - spicy or clove like taste or medicinal taste; esters - a fruity taste; Diacetyls - a butterscotch or "woody" taste. The presence of any of these flavor components depend largely on the style of beer being brewed. Much depends on individual palates and the effect the brewer is aiming for.
Ale yeast ferment at the "top" of the fermentation vessel, at a higher temperature than lager yeast and work quicker. (Ale at 60°-75°F) Ale yeast are Saccharomyces cerevisiae. The average fermentation for ale yeast is 7-8 days. Ale yeast produce by-products of fermentation called esters, the "flowery" aromas of apple, pear, pineapple, grass, hay, plum, and prune that are characteristic of ale.
Lager, (lagern) the German word for "to store", is an excellent description of a beer kept in a cold dark place for 30 days or more. Lager yeast are Saccharomyces uvarum, formerly carlsbergerensis for the place where it was discovered, the lab at the Carlsberg brewery in Denmark. Lager yeast work best at a temperature around 34°F, ferments at the "bottom" of the fermenting vessel and works slowly. Lager yeast produce fewer aromatics than ale yeast and, as a result of the lack of esters, allows the aroma of the hop to be prominent, complementing the sweet flavor of the malt.
Some beer styles are "spiced" with ingredients such as coriander, curacao, all-spice and others.
Adjuncts, other ingredients such as rice or corn, can be used to "extend" the ingredients imparting a "clean" to "no taste" and providing the beer with longer shelf life.