Head cheese is in fact not a cheese, but meat pieces from the head of a calf or pig (sometimes a sheep or cow), in aspic, with onion, black pepper, allspice, bayleaf, salt and or vinegar. It may also include meat from the feet, tongue and heart. It is usually eaten cold or at room temperature as a luncheon meat. It is sometimes also known as souse meat, particularly if pickled with vinegar.
Historically meat jellies were made of the cleaned (all organs removed) head of the animal, which was simmered to produce stock, a peasant food made since the Middle ages. When cooled, the stock congeals because of the natural gelatin found in the skull. The aspic may need additional gelatin in order to set properly.
I just barfed up my P B & J. No offense to those who like eating heads of calf jello.