The executive branch is one of the three branches of a democratic government. Enforcing laws, maintaining the military and running the vast government bureaucracy are all responsibilities of the executive branch.
For most of human civilisation, the executive branch of government has been the dominant one. Dictatorships and monarchies are classic examples of "single-branch" governments that are heavily biased if not totally dominated by the executive branch. The founding fathers of the United States designed the government with checks and balances between the executive, legislative and judicial branches to prevent any one from gaining too much power.
How The Executive Branch Checks Other Branches
In the US, the executive branch has checks over both the legislative and judicial branches. The president can veto bills passed by Congress and can even perform some military actions without the approval of the legislature. The president also appoints federal judges, thus exerting control over the judicial branch. Furthermore, the president can release prisoners, as can state governors.
Checks on the Executive Branch
The legislative branch has the power to impeach and remove officials from the executive branch. The legislative branch also controls the money and can cut off funds to certain parts of the executive branch.
The judicial branch checks the executive branch through the courts. Police officers and state or federal prosecutors are all part of the executive branch, and the judicial branch is responsible for overseeing their behaviour and protecting citizens from unjust prosecution.
Presidential Line of Succession
If the U.S. president dies, the line of succession is as follows (first five): vice president, speaker of the House of Representatives, president pro tempore of the Senate, secretary of state, and secretary of the treasury. The list of succession was established by the Constitution and the Presidential Succession Act of 1947.
The cabinet is composed of 15 advisers who report directly to the president. Cabinet officials are chosen by the president and confirmed by the Senate. Cabinet members are the heads of the various government agencies that make up the government bureaucracy.