In British and Irish cuisine, gammon is the hind leg of pork after it has been cured by dry-salting or brining, and may or may not be smoked. Strictly speaking, a gammon is the bottom end of a whole side of bacon (which includes the back leg); ham is just the back leg cured on its own. Like bacon it must be cooked before it can be eaten; in that sense gammon is comparable to fresh pork meat, and different from dry-cured ham like jamón serrano or prosciutto. The term is mostly used in the United Kingdom and Ireland; other dialects of English largely make no distinction between gammon and ham.
Ham hock, gammon hock, or knuckle, is the back end of the joint, and contains more connective tissue and sinew.