Bitters are, as their name suggest, a bitter or bittersweet type of spirit made from different herbs, roots and plants. Bitters are used to flavor and add a dry zest to cocktails and cooking. Because of their function as a digestive aid and appetite stimulant bitters are often used in before- and after dinner cocktails (aperitifs and digestifs).
When using bitters in cocktails it is important to be cautious as a little too much can ruin the cocktail. Generally, bitters also have a high alcohol content and should never be used in non-alcoholic drinks.