A headquarters unit is a specialised military unit formed around the headquarters of a commanding officer and the requirements of that position. As such, a headquarters unit is always a component of a larger unit.
Examples include: headquarters battalion (controlling a regiment, brigade or larger unit); headquarters company (controlling a battalion); headquarters platoon (controlling a company)
Specific examples include: Headquarters and headquarters company (US Army) Headquarters and service company (US Marine Corps)
References
See also Corporate headquarters