Boston butt is the traditional name in United States. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Some country-style ribs are cut from the upper shoulder (Boston butt) rather than from the loin blade end. Both share the same heavy marbling and long-cook tolerance.
Both are from the shoulder and suited to long, slow cooking, but the butt is fattier and more forgiving while the picnic includes the leg bone and has a stronger flavour.