To quote Wikipedia: A bitters is traditionally an alcoholic preparation flavored with botanical matter such that the end result is characterized by a bitter or bittersweet flavor. Numerous longstanding brands of bitters were originally developed as patent medicines, but are now sold as digestifs and cocktail flavorings in the contemporary market.
Bitters comes in many different flavors and brands. Some of the most famous are Agostura bitters, Bittermens, and Fee Brothers. Fee Brothers brand currently features the following flavors: Aromatic Old Fashioned, West Indies Orange, Peach, Lemon, Grapefruit, Mint, Cherry, Plum, Aztec Chocolate, Rhubarb, Plum, Celery, Black Walnut, Whiskey Barrel-Aged Bitters, and Gin Barrel-Aged Orange Bitters.
THIS ARTICLE from Australian Bartender discusses a tasting event involving bitters and had this lovely and handy explanatory piece with it:
Further historical info from Wikipedia: By 1806, American publications referenced the popularity of a new preparation termed cocktail, which was described as a combination of “a stimulating liquor, composed of spirits of any kind, sugar, water, and bitters.” Of the commercial aromatic bitters that would emerge from this period, perhaps the most well known is Angostura bitters, which was first compounded in Venezuela in 1824 by German physician Dr. Johann Gottlieb Benjamin Siegert. Originally, Dr. Siegert’s bitters was compounded as a cure for sea sickness and stomach maladies, among other medicinal uses. Dr. Siegert subsequently formed the House of Angostura, a company that previously sold the bitters to sailors. Angostura bitters was named after the town of Angostura in Venezuela, although the preparation contains no angostura bark, a medicinal bark which is named after the same town.
Apparently, making your own homemade cocktail bitters is a thing people do! And I sure wish I’d realized this a few months back. I could’ve made some for some fancy-pants, I’m-so-cool Christmas presents for people! Look how pretty her end result turned out!
Now, you may be asking, why did Matty in Training Season drink bitters in his hot water? Well, because they give flavor and they feel a little decadent. I’ve had them a few times when feeling sick and they do settle one’s stomach. I’m not even sure why it came to me that he would do this. I suppose it just seemed like something Matty would do. It’s fancy pants and snooty and feels decadent while not really adding much substance. Very Matty at the beginning of the book, especially.