Amari (literally "bitter" in Italian) are liqueurs (sweetened liquors) that have been bittered and flavored with botanicals. As for the base spirit, well, anything is game: neutral corn spirit, cane distillate, Everclear (popular among home-brewers), even wine. Same goes for the botanical blends — most amari have upwards of 30 botanical ingredients, most often kept secret.
Regarding my quest to pin down the delightfully, decidedly slippery definition and recipe of amaro, I soon realized I was not alone, nor the first.