Campari is an iconic amaro (Italian bitter liqueur) that's probably best known for its role in the classic Negroni recipe, as well as Negroni variations like the Boulevardier and the Old Pal cocktail. It's bright red, very bitter, but quite sweet at the same time—evidenced by the way the cap starts to glue itself shut with sugar after a few pours. It's also often consumed in lighter, low-alcohol cocktails like the Americano, or simply as a Campari and Soda.
Campari was invented in 1860 by Gaspare Campari in Novara, Italy, and it has become one of the largest spirit brands in the world in the century and a half since. Like many other amari, it's a fairly polarizing drink, and the intense bitterness leads most people to agree that you need to try it three or four times before you'll like it. That said, some people (Bevvy staff included) are enamored after their first taste.