Many beer styles are classified as one of two main types, ales and lagers, though many styles defy categorization into such simple categories. Yeasts that ferment at warmer temperatures, usually between 15.5 and 24 °C (60 and 75 °F), form a layer of foam on the surface of the fermenting beer, which is why they are referred to as top-fermenting yeasts, such beers are generally classified as ales. Yeasts that ferment at considerably lower temperatures, around 10 °C (50 °F), have the ability to process a chemical compound known as raffinose, a complex sugar created during fermentation. These yeasts collect at the bottom of the fermenting beer and are therefore referred to as bottom-fermenting yeast. The majority of beer in production today is fermented in this way and is called lager.
Some beers are spontaneously fermented from wild yeasts, for example, the lambic beers of Belgium.
Additional markers are applied across styles. The terms “imperial” or “double” are used interchangeably for a higher-alcohol version of a particular style. Originally applied to Imperial stouts, a high-alcohol style of stout brewed in England for export to Imperial Russia, the term imperial can now be applied to any style name to indicate a higher alcohol content. “Double”, meaning the same thing, originated with the Dubbel style of Trappist beers in the 19th century. Even higher alcohol-content beers can be labeled “triple” (from the Trappist tripel style) or even “quad”. The lower-than-standard alcohol content is often indicated by the term “session”. For example, while India pale ales often have alcohol content around 6-7% abv, a “session India pale ale” will often have alcohol content below 5%.
Originally limited to a few styles in the Low Countries, barrel-aged beer has become more widely applied across just about any style which has had the additional step of being aged in wood barrels; some brewers such as Innis & Gunn market barrel-aged beers which are aged on wood chips rather than inside of wood barrels; wood-chip aging is also used in mass-market beers such as Budweiser.
Recently, brewers have been applying the techniques of traditional sour beers to a wide variety of other styles, by adding additional cultures such as Brettanomyces or Lactobacillus. Wikipedia