Corporal was an enlisted military rank, usually in army units. It was usually higher than a private or trooper, and lower than a sergeant.
In the Imperial Army, a corporal was second-in-command of a nine-man squad, selected and recommended for the position by the sergeant. As the senior of the squad's eight troopers, he was known as the "one-man".
In the real world, the rank of corporal is generally a relatively junior NCO grade.
In the British Household Cavalry, however, all NCOs and warrant officers use variations on the rank of corporal. The rank of Staff Corporal, mentioned in Star Wars Galaxies material, is an element of this system, although only the rank name is actually canon.