Corporal is a military rank. The corporal designation is given to a soldier who has attained the status of a non-commissioned officer, or NCO. A corporal is given the responsibility of overseeing a squad and assuring the squad's readiness for combat or battle. This is the lowest level of NCO and as such, the corporal is often tasked with overseeing clean-up duties as well as physical training, or PT.
In the United States military, the rank of corporal is given many of the privileges of a NCO; the corporal, however, is often just an assistant to a sergeant. The sergeant is responsible for assuring training is completed and typically assigns certain duties to the corporal in order to prepare and train the assistant to advance to the rank of a sergeant. In the U.S. Army, the designation of E-4 is given to a soldier holding the rank of corporal as an NCO. This designation can also be given to a soldier holding the rank of specialist or Spec-4. The Spec-4 is not given the same duties or respect and privilege as that of an NCO.
Enlisted ranks are those from E-1 to E-3: private, private second class and private first class. These are the troops who complete the grunt work and menial tasks of the day-to-day military life. These soldiers are overseen by the NCOs. As an NCO, orders come down from the commissioned officers who are lieutenants and above; the NCO is given the task of making sure the orders from the officers are carried out. The lowest rank in the NCO structure is typically given the job of completing the order and picking the enlisted personnel to carry out the order. That NCO is a corporal.
The sergeant, or E-5, is arguably the work horse of the army. As such, the sergeant is tasked with training raw recruits, creating a fighting force and building troop morale. Given such a daunting task, the sergeant is permitted to have assistants to whom they can delegate some tasks. These tasks are typically given to the lowest in the NCO chain, the E-4. In certain combat situations, the E-4 will be given a squad and will in all matters be treated with the same respect as a sergeant.
The E-4 NCO is perhaps the perfect rank in the military. While not being made to perform menial tasks like an enlisted man or private, the responsible soldier in the event of a foul-up is typically the sergeant or E-5 or above. This affords the E-4 NCO to operate at an almost risk-free manner while gaining valuable leadership experience.