Lager is one of the two main categories of beer, the other being ale.
Lager is brewed with a bottom-fermenting yeast at lower temperatures; Ale is brewed at higher temperatures, with a top-fermenting yeast.
The yeast most often used for Lagers is Saccharomyces carlsbergensis (aka Saccharomyces pastorianus.)
Owing to the lower temperatures, Lagers in general take longer to make than ales.
The flavours of Lager beers are less complex than they are in ales. In general, the taste is less sweet, drier, lighter, and cleaner-tasting.
Currently, the the most widely consumed forms of beer are pale lagers. There are also dark lagers, though.
Lager made in America is largely based on Pilsner-type recipes, and uses corn and rice as additional ingredients to the recipes, besides malted barley. Some say the corn and rice help filter off the excess protein that comes from American six-row barley (compared to the two-row barley prevalent in Europe); some cynics say it is just to reduce cost.
Rice does, though, also make the Lager milder tasting, and thus more appealing to a broader American market. American Lager is very light in colour, with a very mild taste with almost no hop flavour, and an alcohol content of 4 to 6% abv. It can seem bland to non-Americans.
The use of rice in American beers gained popularity during World War Two, when other grains were rationed.
Lagers are usually pasteurised before being shipped to customers, killing off the yeast and making the beer “biologically dead” upon arrival.
Lager is generally best served quite cold.
In the 1400s, Bavarian brewers began storing beers in cool, deep caves to help their stock keep better during summers. Overtime, a yeast evolved that was cold-tolerant. It also acquired the ability to convert sugar to alcohol more effectively, making for drier, crisper beers. The specific yeast culture was first isolated and identified scientifically in 1883 by Emil Christian Hansen, working in the Carlsberg Laboratory in Copenhagen, Denmark. Thus the name, Saccharomyces carlsbergensis, later renamed to Saccharomyces carlsbergensis. The yeast is a hybrid of two different yeasts, Saccharomyces (eu)bayanus and Saccharomyces cerevisiae.
Most lagers were dark ones until the 1840s. Production advances after that made lighter-coloured ones easier to produce.
The first lagers in America were brewed in the 1840s by John Wagner at his brewery on St John Street, near Poplar, Philadelphia. He had done brewing in Bavaria, Germany, and brought the lager yeasts over with him.
A fermentation process known as “continuous fermentation” was developed for lager in 1953 in New Zealand by Morton W. Coutts. This process allows for faster production of lagers, with milder tastes. This allowed the industrialization of the production of light lagers.
Lager means “storage” in German. In German, though, the term just implies storing beer at cool temperatures; it does not imply the bottom-fermentation that it does in English. Instead of asking for a Lager in German, you’d ask for a specific beer.
Dunn, B., and Sherlock, G. Reconstruction of the genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus. Genome Res. doi:10.1101/gr.076075.108.