Beer is a fermented beverage made from grain, yeast, water, and hops.
Beer can be divided into two general categories: ales and lagers. Ales are produced by top-fermenting yeast strains at approximately room temperature, while lagers are produced by bottom-fermenting yeast strains at cooler temperatures. Steam beers, which use lager yeasts at ale temperatures, straddle this divide. Each category can be further subdivided into numerous styles based on their ingredients, brewing process, and place of origin. These styles are based on many subjective qualities and evolve over time. The official style guide published by the Beer Judge Certification Program provides a standard for each style's subjective aroma, appearance, flavor, and mouthfeel, as well as objective characteristics such as ingredients, alcohol content, color, bitterness, and specific gravity before and after fermentation.