Broadly speaking, liqueurs are sweet, flavor-infused spirits. They can be made from fruits, nuts, chocolate, coffee, herbs, spices, or more unusual ingredients like artichoke, Chinese rhubarb, or even tobacco.
Top-quality liqueurs are either produced by fermentation and distillation of the main flavor element or use high-quality spirits. Liqueurs, like cocktail bitters (see below) use a technique called “maceration”; this is, very simply, the infusion of herbs, spices, and other flavor elements into alcohol. Liqueurs are then sweetened with sugar or another sweetener.