Cordials and liqueurs are the equivalent of dessert; usually rich and sweet, often creamy and fattening. The recipe: take a base of pretty much any liquor you choose, and flavor it with just about any other ingredient that intrigues you. Liqueurs range from the frankly medicinal (hello, Jaegermeister) to the sumptuous and decadent (here's looking at you, Chambord and Bailey's).
Like so many types of alcohol, liqueurs got their start as health-giving beverages in the monasteries of Europe. There, monks soaked all kinds of flavorings—fruits, spices, cream—in base liquors ranging from whiskey to brandywine, adding in honey from their own hives to thicken and sweeten. (Whether these tonics were actually medicinal or not is up to debate, but they surely made the middle ages more bearable.)
Whatever your flavor preference, there is doubtless a liqueur to set your tastebuds alight, from creamy to grassy to floral, fruity or minty or even bitter. Many liqueurs are delicious straight up or on rocks; most also make interesting cocktails as well. You'll find them contributing flavor notes to the amazing range of martinis in chic bars today (chocolate, banana, mint) as well as starring in hundreds of fun, flashy, and classic mixed drinks.