This article focuses on the cases where the Head of Government is a separate office from the Head of State.
The head of government is the chief officer of the executive branch of a government, often presiding over a cabinet. In a parliamentary system, the head of government is often styled Prime Minister, President of the Government, Premier, etc. In presidential republics or monarchies, the head of government may be the same person as the head of state, who is often a president (of the republic) or a monarch.
The current record of longest-serving head of government in a democratic nation is Tage Erlander, who was social democratic prime minister of Sweden from 1946 to 1969, a total of 23 consecutive years. He won a total of eight elections and was finally succeeded due to old age.
In semi-presidential systems, the Head of Government may answer to both the Head of State and the legislative power (such as parliament). An example is the French Fifth Republic (1958-present), where the Président de la République appoints a Prime Minister but must choose someone who can get government business through, and has the support of, the National Assembly. When the opposition controls the National Assembly (and thus government funding and most legislation), the President is in effect forced to choose a Prime Minister from the opposition; in such cases, known as cohabitation, the government controls internal state policy, with the President restricted largely to foreign affairs.