Chairman of the Joint Chiefs of Staff
In the United States Department of Defense, the Chairman of the Joint Chiefs of Staff (CJCS) is the senior uniformed officer of all military services. While he is the senior officer, however, he does not directly control any military forces, but acts as the principal military advisor to the National Command Authority, oversees all military planning, and often is involved policy discussions with Congress.
The Chairman is assisted by a four-star Vice Chairman, and the Joint Staff reports to him through a three-star Director; they are the secretariat and planning group for the Joint Chiefs of Staff. ADM Mike Mullen is the incumbent Chairman and GEN James Cartwright, USMC, is the Vice Chairman.
This position, as well as the formal organization of the Joint Chiefs of Staff, was created by the National Security Act of 1947. That legislation made the CJCS a statutory advisor to the National Security Council, although not a voting member of the NSC. Under the U.S. system of civilian control of the military, the United States Secretary of Defense actually is the head of the Department of Defense, and, should matters come to a vote, is the representative of the Department.
Originally, the individual uniformed service chiefs were equals, with the Chairman chairing their committee and having his own staff, but being on a par with the:
- Army Chief of Staff
- Chief of Naval Operations
- Air Force Chief of Staff
- Commandant of the Marine Corps
... the principal military adviser to the President, the National Security Council, and the Secretary of Defense...After first informing the Secretary of Defense, a member of the Joint Chiefs of Staff may make such recommendations to Congress relating to the Department of Defense as he considers appropriate.
This did not, however, put the Chairman in the regular line of command. The Chairman and his staff may assist the passage of orders from the National Command Authority to the Unified Combatant Commanders. Goldwater-Nichols made it clear that the operational chain of command runs from the President, to the Secretary of Defense, to the commanders of the geographic (e.g., Pacific Command) and functional (e.g., Strategic Command) Unified Combatant Commands (UCC). 
Appointment and qualifications
"The Chairman serves at the pleasure of the President for a term of two years, beginning on October 1 of odd-numbered years. ... an officer serving as Chairman may be reappointed in the same manner for two additional terms. However, in time of war there is no limit on the number of reappointments." 
- United States Department of State, National Security Act of 1947
- Goldwater-Nichols Act of 1986, 10 US Code 151-155
- Goldwater Nichols Department of Defense Reorganization Act of 1986, National Defense University Library