Specifically, bocconcini are spongy little balls of mozzarella. Originally they were made only with buffalo milk, but now they are made of both cow and buffalo, or even just cow. Bocconcino di Bufala Campana DOP is the mini version of the Mozzarella di Bufala Campana.
Beyond that, bocconcini can be little balls of just about anything: meats, other cheeses, fish, potato, aubergines, etc, used as antipasti.
Images by: Larry Hoffman