I love the education! "Pork shoulder" is how it is wrapped and sold in my local grocery stores -- had no idea that pork butt was a part of the shoulder. Who knew?! (okay, maybe I'm the slow one in class...)

I couldn't have said it better than usuba dashi just did.

Personally I'm a big fan of the picnic portion.

A pork shoulder consist of 2 parts - Pork butt (the best part of the shoulder, the bit of muscle before the loin, containing the shoulder blade and nice fat marbling)) and the picnic (think of it as the "arm" of the pig). In the meat business, if I order pork shoulders, I will get the butt and shoulder together as one piece.

I always use a pork shoulder -- as taught by one of my Southern friends...

Butt is best, though a picnic can work (more sinew) and even a fresh ham can work. Butts are always the best though.