Together the butt and the picnic make up a "whole shoulder," which includes the shoulder and upper foreleg above the knee. The whole shoulder is made into two separate cuts, butt and picnic, by the simple expedience of sawing across the upper foreleg. The butt is the portion including and closest to the shoulder itself, the picnic the portion closest to the knee.
A bone-in butt is fairly rectangular; while the picnic is more cone shaped, like a ham. In fact, it's often called a picnic ham. The picnic includes a slightly higher proportion of bone. Another difference is that when the picnic is sold whole (whole picnic) there's usually some skin attached.
Aside from their varying geometries and the challenges imposed by working around their bones, butt and picnic cook similarly. The cook should recognize that both cuts are full of connective tissue and fat pockets, and that the meat is relatively tough. Low temperature and long cooking, whether braising, stewing, barbecuing or slow-roasting are most favored.
The lower foreleg, from knee to the ankle is called the shank. The only legitimate use for the shank is eisbein. Don't be deflected by other recipes. If you do make eisbein, let me know and I'm there.
Ankles and feet, up to the hooves are "trotters." They make beer taste better.
Hope this helps,