Pork butt, despite its colorful name, does not come from anywhere near the butt or behind of the pig. In fact, quite the opposite. Pork butt is a cut of meat from the shoulder of the pig. Seriously.
Pork Butt Versus Pork Shoulder
So where does the cut of pork called pork shoulder come from? Also the shoulder.
Technically "pork butt" (as well as Boston butt, a different name for more or less the same cut of pork), comes from the thicker section of the shoulder where there is more intense marbling, or fat running through the meat. It may contain the shoulder blade bone but usually doesn't.
The name makes sense when one remembers that the word "butt" can also mean the thicker end of something (the butt of a gun) or the blunt end of something (the butt of a cigar), since a pork butt is the thicker end of the shoulder cut.
Cuts labeled "shoulder" (including a "picnic shoulder") are from the thinner, triangle-shaped end of the shoulder, which would be attached to the butt if they weren't commonly separated into smaller cuts. It has less marbling and less fat.
How to Cook Pork Butt
Both pork butt and pork shoulder cuts do best with long, slow cooking and are excellent choices to be barbecued, braised, or used as stew meat, as well as roasted or cooked in slow-cookers. They also stand up well to strong flavors (think barbecue sauce or chiles) because they have a strong flavor themselves.
This is because they are from a hard-working section of the body—carrying around all that hog weight and developing great flavor—this is notably true of pastured pork from pigs raised in environments where they can walk freely.
In short, you can, if necessary, use pork butt and pork shoulder interchangeably in most recipes. Pork shoulder is a bit better if the final plan is to slice or chop the meat and have it hold its shape, while the more intense marbling of pork butt makes it particularly well suited for barbecue, specifically making pulled pork or other recipes where you want the final dish to fall apart into shreds easily.
Pork butt is delicious in spicy stews such as New Mexican Carne Adovada, this Green Chili, or a classic Posole since the fat in it helps really spread that chile flavor around, and the chunks of meat will tend to get even more tender than pork shoulder does. Again, it's all about the marbling. Butt is also used to make carnitas and pulled pork.
Wait, But What About the Actual Butt?
That area of the pig we might think of the actual butt? The behind? The bottom? The big muscle at the top of the leg? That's where the ham—cooked fresh or cured or smoked—is from. Ham is the thigh and most of the gluteus maximus (that's why it's so meaty).
Note: Many styles and regional variations exist in butchery, so when in doubt, ask your butcher for specifics.