| There are two types of command personnel in any unit larger than a squad, a comissioned officer and a non-commissioned officer (NCO). The primary difference betwen the two is that the Commissioned Officer holds an assignment to his rank from the government, specifically the President of the United States. (Warrant Officers are by design created to operate outside of the chain of command, combining characteristics of both types of rank.)
This means that an officer is a direct representative of the United States and can act in her name within the limits of the orders he or she has been given by superior officers. A non-commissioned officer holds their rank by appointment of the department of defense, and therefore are subordinate.
The difference in authority and responsibility between the two types of rank is huge. A commissioned officer has the power to kill in the name of the government. The enlisted soldier only kills on command, he is not "responsible" for the aim point of his gun. An officer also has the authority to use deadly force on their subordinates.
This is also why military aircraft are flown by officers, as they are an independent command. Even Army helicopter pilots are warrant officers to sidestep this issue.
An NCO has his or her authority from the officer above them in the chain of command. A sergeant can only tell a soldier what to do within the authority of their position. | 
