Chinotto is a small, bitter citrus fruit common to southern Italy. It's an important flavor compound in many Italian bitters and, along with some herbal extracts, is also the basis for a strongly-flavored cola-like drink of the same name. There are several brands of chinotto sold in Italy, but the most common one found in the US is by San Pellegrino. I love the stuff. The rest of my family hates it.
Some people compare it to Coca-Cola, but that's not really a fair comparison to either beverage. Coke is sweet and lighter in character; chinotto is bitter, with herbal notes and a hint of citrus behind it all. If I were to compare it to anything it would be Moxie - that old New England favorite that is, much like chinotto, an acquired taste.