Contrary to what their name suggests, sweetbreads are not sugary pastries.
Rather, sweetbread is the culinary name for the thymus and pancreas glands of an animal —generally a calf, lamb or piglet less than a year old.
The thymus is an organ located in an animal's neck, and is responsible for producing new T-cells, which are important for proper immune function. It develops prenatally (before birth), but starts to disappear after the animal is no longer drinking milk; as the animal matures, new T-cells are produced through the division of mature T-cells. (In humans, the thymus begins to shrink and becomes less active after puberty).
Sweetbreads produced from the thymus are classified as either heart sweetbreads or throat sweetbreads, which differ by their shape and what part of the thymus they come from. Heart sweetbread is the larger and rounder of the two, whereas throat sweetbread is somewhat cylindrical.
Less often, sweetbreads come from the pancreas — a gland that produces insulin and other important hormones — of young animals, in which case they are typically referred to as stomach sweetbreads.
In culinary art, sweetbreads are classified as offal (the internal organs or entrails of a butchered animal) and are often considered gourmet food. They are prepared in a number of ways, such as breaded and fried, braised or sautéed.