Sweet breads are made from the pancreas and thymus glands. They can come from any meat animal, but most commonly they are made from lamb or calf.
The pancreas is located in the chest or abdomen, and the thymus glands are located in the neck. According to Serious Eats, sweet breads are most commonly referred to as "heart sweet breads" if they come from the pancreas, and "throat sweet breads" if they come from the thymus glands. The glands themselves are eaten as part of sweet bread, but the tough membranes are not usually included. Sweetbreads have been described as tender, creamy and juicy.