What is a lager beer?

Lager is brewed with a ‘cold-fermenting’ yeast, which sinks to the bottom during fermentation. They take longer than ales to mature, and must be stored (lagern means ‘to store’ in German) in cold temperatures before going into bottles or kegs. While there are dozens of styles of lager they usually tend to have a crisp bitterness and subtle flavours and aromas and are best served a little colder (4-7 degrees Celsius) than ale. Lager is by far the most popular style of beer commercially.
Two common varieties of lager are Czech Pilsners (traditionally from Pilsen in Czech Republic) and German Helles (meaning light in colour). Most commercial lagers (1664, Fosters, Heineken etc) are based on one of these styles. The primary difference between the two traditional varieties is that Helles tend toward a softer bitterness and more malty flavours whereas Pilsners exhibit more crisp bitterness. Two of the biggest selling craft beers in the world today, Brooklyn lager and Sam Adams are actually both a type of traditional lager known as Vienna lager.