[quote=In the US, what does a Chief of Staff Army do and how is it different from the Joint Chief of Staff of the Military? Does JCS outrank Chief of Staff Army and thus can order him around?[/QUOTE]
Old thread...not sure if you got this info yet so I'll give it a shot.
The Chairman of the Joint Chiefs of Staff is the senior military advisor to the President and the Secretary of Defense. He chairs the Joint Chiefs which is composed of the Vice Chief, the Chief of Staff of the Army, the Chief of Staff of the Air Force, Chief of Naval Operations and Commandant of the Marine Corps.
Each of the service chiefs mission is to put their service "face" to the JCS and advise the president. The also testify to Congress, etc. They, along with their service secretaries, establish policy and procedures for their services to follow IAW guidance from the Secy of Defense. The vice chief of staff manage the day to day running of the Army through the Army Staff.
As for command channels the Unified Combatant Commands are responsible to the President and the Secretary of Defense for accomplishing the military missions assigned to them. Commanders of the Unified Combatant Commands exercise command authority over forces assigned to them as directed by the Secretary of Defense. The operational chain of command runs from the President to the Secretary of Defense to the Commanders of the Unified Combatant Commands. The Chairman of the Joint Chiefs of Staff functions within the chain of command by transmitting to the Commanders of the Unified Combatant Commands the orders of the President or the Secretary of Defense. Unified Combatant Commands include the European Command, Pacific Command, Joint Forces Command, Southern Command, Special Operations Command, Strategic Command, Central Command, Transportation Command, and Space Command.
DoDOrgStructure