In the
US Army, every military unit larger than a squad (specialty units are an exception) has at its head an enlisted/officer pair.
In a platoon, made up of 4 or 5 squads, the head officer is a 2nd (often called a "butter-bar" for the gold of the insignia and the incredible lack of experience) or 1st Lieutenant (O1-O2) and the senior enlisted is a buck or staff sergeant (E5-E6).
In a company, there is a 1st Lt. or Captain (O2-O3) with a Sergeant First Class or Master sergeant (E7-E8). A Master Sergeant in charge of a company gets a diamond-shaped mark in the middle of their rank insignia to designate them as "First" Sergeant. (they are often called "Top" as a nickname.) A 1st Lieutenant (often called a "one-el-tee") has a single silver bar, a Captain, two.
Officers up to the rank of Captain are called company-grade officers, for obvious reasons.
In a battalion, usually made up of 3-5 companies, there is a Major (or a very, very up-and-coming Captain) or a (not very up-and-coming) Lieutenant-Colonel, (also called a "Light" Colonel) (O4-O5,)with a Master Sergeant or Sergeant-Major (E8-E9). A Sergeant-Major has a star in the center of their insignia. One in charge of a unit is called a Command Sergeant-Major and has a wreath around that star.
Majors and Colonels are called "Field-Grade" officers, as they manage multiple units on the field of battle. Majors have a gold oak leaf, and Light Colonels a silver one.
Full Colonels (O6, the equivalent of a Government GS-15) have a silver eagle on each shoulder, hence their nickname, "Bird Colonel".
At higher levels, the officer ranks go up, but the enlisted ranks are capped at Sergeant-Major. From this point, they are referred to by the size of the unit they are the senior enlisted of, up to and including the Sergeant-Major of the Army (he gets to have two stars in his insignia.)
Generals are called "Flag" officers, as they actually have their own flag. This dates from the days when the only way to communicate on the battlefield was with drums, horns, or messengers, and the troops needed to know where their commander was. The flag carries the number of stars the General (Or Admiral, this also applies to the Navy) has, and is displayed on the building they are currently occupying and on their vehicle. (You have to salute the vehicle when it drives past.)