Pay grade E4 through E5. Corporals through Sergeants is the standard (it can be different in the Coast Guard or Air Force.) It only applies to enlisted military personnel not officers. Officers have a different ranking structure (example: O4 through O5) and have gone to college or some form of higher education.
Naval academies and the like count.
Most of the time they were in ROTC. Unless they are a Warrant Officer, in which case, they started out enlisted then switched over to Commissioned Officer, usually sacrificing some pay
NCOs are usually put in charge of squads or fireteams but not entire platoons as that is up to the Staff NCOs. They're in charge of Field Days but still have to clean. They can lead a run but not get out of one. They have to go on every patrol but they get to decide when to rest. A good example of someone who has responsibility but very little authority.
In the USMC, NCOs rate a blood stripe and a sword.