Most democracies in the world have a parliamentary system. In this system, the executive and legislative branches of government are combined and the political head of state is chosen from within the legislature. This political head of state is usually called a prime minister, and he or she is chosen by the majority party in Parliament to serve as the head of the majority party. 

The combining of executive and legislative branches can be both an advantage and a disadvantage. Some scholars and observers would argue that it is easier to pass laws in a parliamentary system as the head of state is always chosen from the majority party, so a divided government is generally not an issue. Others would take issue with the fact that prime ministers are neither directly elected by the people, nor able to take a popular stand against the majority of Parliament (because they would be simply removed through a no-confidence vote if they were to lose the support of their party).

This system is distinguished from others because it has a chief executive (the president) who is chosen by the people to serve a limited term in office with a distinct separation of powers (the executive branch) as well as specific limitations on exactly what he/she can do while in office.

The president serves not only as head of state but he or she is also in charge of the executive branch of government. He or she has the power to appoint members of his/her executive cabinet to oversee major bureaucratic departments within the government, serves as the civilian head of the armed forces, and is responsible for setting foreign policy as well as determining and influencing domestic policy and legislation.

An advantage of presidential systems may be that the powers of a president are balanced by a legislature, which is not only popularly elected but also acts independently of the president. In many cases the president may be from a different party than the majority of one or both houses of Congress; the only way the president can get his/her policy agenda made into law is by cooperating and compromising with Congress and vice-versa.