Intensely bitter since its inception in 1845, Fernet-Branca has been produced according to the original recipe that has been handed down from generation to generation. The bitters are made from 27 herbs and other ingredient. The exact formula is a trade secret known only to the Fernet-Branca president, Niccolò Branca, who personally measures out the aromatics during the production process. Fernet-Branca has enjoyed a recent growth in popularity in the United States particularly due to an increase in the interest in and mixing/serving of 'vintage' cocktails.
The AV Club has a wonderful writeup on Fernet:
Fernet’s many flavor layers make it a great addition to cocktails in need of a little kick, but it’s delicious on its own, too. The first wave you’ll get is that bitter mintiness, followed by a more balanced taste that’ll leave you wondering: What was all of that? It was 25-plus herbs and spices working together.