Some of the different kinds of bitters used behind the bar are:
Angostura Bitters was first compounded in Venezuela in 1824 by a German physician, who intended it as a remedy for stomach maladies. It was exported to England and to Trinidad, where it came to be used in a number of cocktails. Angostura and similar gentian bitters preparations are still of some value to settle a mild case of nausea or an upset stomach. Their use in cocktails, however, accounts for the vast majority of the sales. Angostura was named for the town of Angostura in Venezuela, and contains no angostura bark.
Peychaud's Bitters is associated with New Orleans, Louisiana, and can be difficult to find elsewhere. It, too, is a gentian based bitters, with a subtly different and sweeter taste than the Angostura brand. Peychaud's Bitters is associated with the Sazerac cocktail.
Orange bitters are made from the rinds of unripe oranges. They are called for in some older cocktail recipes.