군인 would mean someone who's serving in the military. This includes everyone in uniform from a private to a general or an admiral. 예비군 a reserve, non-thinker I guess 병사 This is the term that you'd want to use when you're referring a lower-ranked 군인, such as a private, a private first class, a corporal, or a sergeant.

군인 a person who serves in an army; a person engaged in military service. It includes all kind of commissioned officers and non-commissioned officers. 예비군 a person who belongs to a reserve military force of a country; a reserve military force of a country. 병사 an enlisted man or woman, as distinguished from a commissioned officer.