The main difference between a presidential and a parliamentary system of government lies in the relationship between the executive and legislative branches. In a presidential system, the President serves as both head of state and government, elected independently of the legislature. This separation of powers ensures a distinct executive authority. In contrast, a parliamentary system integrates the executive and legislative branches, with the Prime Minister emerging from the majority party in the legislature, leading to more collaborative governance. This distinction affects policy-making and accountability mechanisms within the respective systems.