Now that craft cocktails have gained their place in the world of drinking—and are no longer relegated to girls' night out or bastardized versions of the classics—it's fitting that home bartenders should focus on bitters. It was once enough when asked about bitters to point to a crusty bottle of Angostura on the back bar and be done with it, but now the game has changed.
The first definition of the cocktail in print appeared in The Balance and Columbian Repository in 1806, which lists four essential ingredients: liquor, sugar, water, and bitters. However, bitters predate the cocktail. Bitters were sold as curative formulas and, frankly, were nothing short of snake oil. Early advertisements for the now defunct Pond's Bitters claimed that the bitters cured headaches, dyspepsia, biliousness, diarrhea, and constipation. (Presumably the bitters know when to put on the gas and when to put on the brakes for the latter two ailments—no pun intended.) Yet enterprising bartenders decided that these elixirs work well in drinks.
As you can well imagine, the essential characteristic of a bitter is that it contains a bittering agent. These agents are generally macerated in alcohol—a solvent—and sometimes in water and glycerin with stabilizers. Common bittering agents, and ones that I use in my own recipe, include gentian root, quassia bark (quassia amara contains quassin, allegedly the most bitter substance on the planet), quinine, wormwood, and dandelion.