A presidential system of democracy is based on the separation of powers between three branches of government: judicial, legislative and executive. The United States originated the use of a presidential system, and only a few other democracies follow the format.
The president is the head of state, and oversees the executive branch of the government. To prevent the president from having too much power and abusing it, the judicial and legislative branches provide checks and balances against the executive branch's powers. Presidents and other members of the executive office can be dismissed through a process of impeachment and conviction if the president is found unfit to lead.