I have eaten more head cheese this year than I have in every other year of my life, combined. I always secretly hope that it isn't as, well, heady as the name sounds, but I always know, deep down, in-between chews, that I am eating throw away cuts or, if not, cuts that need more than just a brine and puree. I dislike it texturally and for its flavor, but have no problem with it conceptually, nor will I turn away cuts of tongue or cheek, but at least those have an obvious origin. Head cheese is a catchall, like someone telling you that your deli meat is turkey, beef, or ham. Those define meat from a specific animal, but there is plenty of meat on each animal that I wouldn't touch.

From the Wikipedia entry:

Head cheese (AmE) or brawn (BrE) is a cold cut originating in Europe. Another version pickled with vinegar is known as souse. Head cheese is not a cheese but a meat jelly made with flesh from the head of a calf or pig (sometimes a sheep or cow) in aspic. While the parts used can vary, the brain, eyes and ears are often removed. The tongue, and sometimes even the feet and heart may be included. Head cheese may be flavored with onion, black pepper, allspice, bay leaf, salt, and vinegar. It is usually eaten cold or at room temperature as a luncheon meat.
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.