Beer is the world's oldest and most popular alcoholic beverage, selling more than 133 billion litres (35 billion gallons) per year. It is produced through the fermentation of starch-based material, commonly barley, though cassava root in Africa, potato in Brazil, and agave in Mexico, among other starch sources, have been used.

Alcoholic beverages made from the fermentation of sugars derived from nonstarch sources (e.g., grape juice or honey), and beverages which are distilled after fermentation are not considered to be beer.

The characteristics of beer such as taste and colour vary according to the ingredients and procedures used to make it. While local names for beers made with the same methods and ingredients vary, the similarities of method and ingredients can be detected to form a study of the nature of beer styles.