Pork shoulder and pork butt are often called for in recipes, and while they sound like they come from opposite ends of the animal, they are actually in close anatomic proximity.
Pork butt gets its name from how the meat was once packed. In colonial New England, it was stored in large drums called butts and is still sometimes labeled as Boston butt.
Pork shoulder, on the other hand, is often labeled as picnic shoulder. The two cuts are right next to each other on the pig’s shoulder, with the butt on top of the picnic shoulder.
The pork butt is a high-fat cut, so it isn't as prone to drying out over a long cooking period. Pork butt can be sold bone-in, but is often sold bone-out as well.
Picnic shoulder is a leaner cut that’s usually sold with the bone in and almost exclusively with the skin on. It comes in a triangular shape with one end tapering down.
A pig's shoulder gets worked out regularly and has thick muscle strands that break down and become tender when cooked slowly, unlike lean cuts that need to be cooked quickly.
Pork shoulder and pork butt have a deeper flavor than the leaner parts of the animal. Picnic shoulder is a bit leaner, stringier, and less savory, but its skin can become crispy.