Here is a pretty fair description from Hormel.
Primal Cut - Shoulder
The shoulder is the primal cut that includes the front leg and the section at the top of the leg. It contains a higher level of fat than the other cuts of pork, which provides it with a lot of flavor and tenderness, but also causes the cuts from this area to add more fat into our diets than the meat from some of the other primal cuts. The fat content in the shoulder makes this cut desirable for making sausage and when well-trimmed, it is used for lean ground pork and is also cubed or cut into strips to use for kabobs, stir-frying or stewing. The shoulder is one of the most flavorful and economical cuts.
Subprimal Cut Market Ready Cuts Description
Blade (or Butt) Shoulder Blade Boston Roast,
Smoked Shoulder Roll,
Blade Steaks,
Cubes The blade shoulder subprimal is a better cut than the picnic. The blade is the upper portion of the shoulder and is tender and full of flavor. The roasts from this cut are available bone-in or boneless and are best cooked using a moist heat method, such as braising or stewing, but they can also be roasted. The steaks, which are cut from the blade Boston roasts, are best broiled, grilled or braised.
Picnic (or Arm) Shoulder Picnic Roast,
Arm Steaks,
Picnic Ham,
Shanks (Hocks),
Ground Pork The picnic shoulder is more economical but also contains more fat than the blade shoulder. When the bone and fat is trimmed from this cut it results in a very rich flavored roast. The meat from this cut is excellent for making juicy barbecued pulled pork. Many times the picnic shoulder is cured and smoked to make a picnic ham, which is ready to eat when purchased or can be heated before eating if desired. If not cured and smoked, they can be roasted, baked, braised, or simmered. The fresh meat from this cut is best cooked by methods such as roasting, braising or stewing. Steaks are especially good tasting when grilled.
Depending on the store, bone in shoulder and boston butt could be the same.
Very few stores would carry a whole shoulder 16-20- lbs.
The shoulder would be the front leg of the hog.
The upper portion,with the blade bone, would be the "Boston butt" and the lower portion ,towards the trotter, would be the picnic shoulder.
The picnic tends to have more tendon/ligament,,etc than the butt.
Picnics may sell for 1/2 as much per pound.
Some folks think the picnic is "hammier" tasting.
The butt is typically taken up to around 195º internal and pulled.
Picnics can be,but are often taken off about 185º and sliced like a ham.
Hope this helps a little.