Bitters are alcoholic beverages heavily flavoured with botanicals. Originally created to harness the properties of botanicals for digestive ailments, they have become more commonly used as a flavouring agent in cocktails, and also as digestifs.
As a cocktail agent they are receiving a revival in interest with many new artisan brands appearing on the market, and an interest in reviving lost brands.
Once a hugely popular category, their usage died out somewhat, with only the largest brands surviving the double blow of prohibition and changing tastes, and the use of medical advances in aiding digestion.
Due to the highly bitter nature of the products, in cocktails they are often used sparingly, a dash or two adding complexity and balancing sweetness with bitterness. These cocktail bitters are usually sold in small bottles and come fitted with a dasher on the top, such as Angostura, Peychaud or Fees. Common ingredients include gentian root, quinine, dried orange peels, angostura bark.
There are many somewhat sweeter bitters that could also be classed as liqueurs or digestifs or even vermouths such as Carpano Punt e Mes, Cynar, or Bercherovka. These are usually consumed as an after dinner drink to aid digestion.