The power of the Executive Branch is vested in the President of the United States, who also acts as head of state and Commander-in-Chief of the armed forces. The President is responsible for implementing and enforcing the laws written by Congress and, to that end, appoints the heads of the federal agencies, including the Cabinet. The Vice President is also part of the Executive Branch, ready to assume the Presidency should the need arise.

The Cabinet and independent federal agencies are responsible for the day-to-day enforcement and administration of federal laws. These departments and agencies have missions and responsibilities as widely divergent as those of the Department of Defense and the Environmental Protection Agency, the Social Security Administration and the Securities and Exchange Commission

source: whitehouse.gov