A drill instructor is a non-commissioned officer

Non-commissioned officer

A non-commissioned officer , called a sub-officer in some countries, is a military officer who has not been given a commission...

 or Staff Non-Commissioned Officer in the armed forces

Armed forces

The armed forces of a country are its government-sponsored defense, fighting forces, and organizations. They exist to further the foreign and domestic policies of their governing body, and to defend that body and the nation it represents from external aggressors. In some countries paramilitary...

 or police forces with specific duties that vary by country. In the U.S. armed forces

Military of the United States

The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.The United States has a strong tradition of civilian control of the military...

, they are assigned the duty of indoctrinating new recruits

Recruit training

Recruit training, more commonly known as Basic Training and colloquially called Boot Camp, is the initial indoctrination and instruction given to new military personnel, enlisted and officer...

 entering the military into the customs and practices of military life. In the U.S., a drill instructor refers to a Marine Corps Drill instructor. In the Air Force they are known as Military Training Instructors.