A non-commissioned officer (NCO) is a person, in military service, who supervises individuals in the performance of their duties, or may be a higher-level supervisor of people who work directly with individuals. Professional NCOs have a role that is complementary to that of commissioned officers. An NCO is responsible for the preparation of individuals carrying out unit tasks and enabling their correct performance of individual responsibilities, while a commissioned officer is responsible for the preparation of the unit and its execution of tasks.
Effective NCOs usually rise from the ranks of the type of individual they supervise, and can coach and direct based on personal experience. As NCOs gain more experience, depending on the particular military organization, they may focus on managing larger groups of individuals and supervisors, or may choose to become increasingly knowledgeable in some skill set valuable to the unit, serving as technical experts.
While the U.S. military during the Vietnam War, and the Soviet Red Army throughout its existence, tried to accelerate NCO development by taking selected recruits and putting them through a special leadership school, such an approach did not work well for leaders of individuals; experience with the tasks being supervised proved necessary.
During World War Two, however, the U.S. Navy did find that it could be useful to give NCO rank to recruits with significant civilian experience in the field, such as carpenters or heavy equipment operators in the "Seabees" U.S. Navy Construction Battalion. This approach still respected the need for experience, but there were few civilians indeed that had experience with direct combat skills.
In ground and air forces, NCOs are usually called sergeants, often with a prefix such as "master sergeant" (U.S.), "company sergeant major" (U.K.) or "senior sergeant" (Russia). It will vary with the service if a corporal is considered an NCO. For those NCOs that choose to specialize as technical experts, there may be different terminology for the supervisory and technical paths: in the United States Marine Corps, the highest supervisory-track rank is sergeant-major, while the highest technical-track rank is master gunnery sergeant.
Naval services use different terminology, such as "petty officer" at the mid-level ranks and "chief petty officer" at the higher ones. The U.S. Navy differentiates between "rating", or the type of work one does, and the "rank". An individual might be a "gunner's mate second class", indicating one is a specialist in gunnery of moderate rank, or "master chief yeoman", indicating the highest rank in administrative functions.