The tradition of the Cabinet dates back to the beginnings of the Presidency itself. Established in Article II, Section 2, of the Constitution, the Cabinet's role is to advise the President on any subject he may require relating to the duties of each member's respective office.
The Cabinet includes the Vice President and the heads of 15 executive departments — the Secretaries of Agriculture, Commerce, Defense, Education, Energy, Health and Human Services, Homeland Security, Housing and Urban Development, Interior, Labor, State, Transportation, Treasury, and Veterans Affairs, as well as the Attorney General.
In order of succession to the Presidency:
Vice President of the United States
Joseph R. Biden
Department of State
Secretary John Kerry
U.S. Department of State
Department of the Treasury
Secretary Jack Lew
U.S. Department of the Treasury
Department of Defense
Secretary Chuck Hagel