In political systems based on the principle of separation of powers, authority is distributed among several branches (executive, legislative, judicial) -- an attempt to prevent the concentration of power in the hands of a small group of people. In such a system, the executive does not pass laws (the role of the legislature) or interpret them (the role of the judiciary). Instead, the executive enforces the law as written by the legislature and interpreted by the judiciary. The executive can be the source of certain types of law, such as a decree or executive order. Executive bureaucracies are commonly the source of regulations.
In the Westminster political system, the principle of separation of powers is not as entrenched. Members of the executive, called ministers, are also members of the legislature, and hence play an important part in both the writing and enforcing of law.
In this context, the executive consists of a leader(s) of an office or multiple offices. Specifically, the top leadership roles of the executive branch may include:
In a presidential system, the leader of the executive is both the head of state and head of government. In a parliamentary system, a cabinet minister responsible to the legislature is the head of government, while the head of state is usually a largely ceremonial monarch or president.