Fernet-Branca is an Italian amaro liqueur that's bitter, minty, and bright. It's made from a secret blend of "dozens" of herbs and spices (not the least of which is saffron, which the distillery purchases in larger quantities than any other entity on earth), and is a perennial favorite in Italy, Argentina, and of course, San Francisco.
Though Fernet-Branca is the best known of its category, fernet actually comprises an entire range of amari. Fernet is generally very bitter, minty, and thanks to the eclectic array of ingredients, a truly unique flavor all around. Fernet-Branca is frequently quaffed with a ginger ale back in San Francisco, in a Fernet and Coke in Argentina, and often mixed with soda water in Italy, much like Campari or other amari.
Fernet-Branca is an extremely divisive product, with fans and evangelists swearing by it as everything from a delicious digestif to a health tonic, and detractors insisting that it's nothing more than "mouthwash with delusions of grandeur." As with so many other bitter Italian liqueurs, the only way to know for sure which side you're on is to try it yourself—at least three times, according to tradition.