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From Wikipedia: Joint Chiefs of Staff:

The Joint Chiefs of Staff (JCS) is a group of military leaders in the United States armed forces who advise the civilian government of the United States. The JCS is defined by statute and consists of a Chairman and Vice Chairman appointed by the President, and the Chiefs of service from four of the five branches of the armed services.

From Wikipedia: Chairman of the Joint Chiefs of Staff:

The Chairman of the Joint Chiefs of Staff (CJCS) is by law the highest ranking military officer in the United States armed forces, and the principal military adviser to the President of the United States, the National Security Council, and the Secretary of Defense. The Chairman outranks all respective heads of each service branch but does not have operational command authority over them or their service branches. He leads the meetings and coordinates the efforts of the Joint Chiefs of Staff (JCS), comprising the Chairman, the Vice Chairman of the Joint Chiefs of Staff, the Chiefs of Staff of the United States Army and United States Air Force, the Chief of Naval Operations, and the Commandant of the Marine Corps. The Joint Chiefs of Staff have offices in The Pentagon.

Although the office of Chairman of the Joint Chiefs of Staff is considered very important and highly prestigious, neither the Chairman, the Vice Chairman, and the Joint Chiefs of Staff as a body have any command authority over combatant forces. The Goldwater-Nichols Act places the chain of command from the President to the Secretary of Defense directly to the commanders of the Unified Combatant Commands.However the chiefs do have authority over personnel assignments and oversight over resources and personnel allocated to the combatant commands within their respective services. The Chairman may also transmit communications to the combatant commanders from the President and Secretary of Defense as well as allocate additional funding to the combatant commanders if necessary.