The duties of the White House Chief of Staff can vary greatly from administration to administration, but generally he (and all have been male, to date) is responsible for overseeing the actions of other members of White House staff, managing the president's timetable, and controlling outsiders' access to the president. This last duty has earned the Chief of Staff the nickname of "the gatekeeper." Informally, the Chief of Staff is often one of the President's closest political advisers, and also often a close personal friend.
Not every President has had a formal Chief of Staff. Kennedy did not, and Carter didn't until the very end of his term.
Most White House Chiefs of Staff are former politicians, and many continue their political careers in other senior roles. Examples include Richard Nixon's Chief of Staff Alexander Haig who later became United States Secretary of State, Gerald Ford's Chiefs of Staff Dick Cheney, now Vice President of the United States, and Donald Rumsfeld, who later became United States Secretary of Defense.
Some have suggested that a powerful Chief of Staff dealing with a "hands-off" president who decides not to become involved in the minutiæ of government, can become a quasi-Prime Minister. Such prime ministers exist in some presidential systems, such as in France and Russia, with the prime minister running the government and the president remaining somewhat aloof from the political process but setting broad policy goals. James Baker and Donald Regan were seen as prime ministerial-style chiefs of staff during the Reagan presidency. Howard Baker, who succeeded Regan, was critical of this system and what is sometimes called the Imperial Presidency.
By contrast, Andrew Card, the current Chief of Staff in the administration of President George W. Bush, is not regarded as a very powerful figure, in large part because Bush appears to deal directly with his Cabinet secretaries. Similarly, President Clinton's Chiefs of Staff were not particularly powerful under his watch, because Clinton's workaholic personality kept him in constant touch with the government.
The Chief of Staff is assisted by one or more Deputy White House Chiefs of Staff. Karl Rove and Joe Hagin currently have this title.