Compare and contrast federal and unitary governments? How is power differently dispersed in parliamentary, presidential, and semi-presidential systems?

What will be an ideal response?

Some constitutions—such as those of France or Japan—are unitary systems, which means that the central government has veto power over the decisions of subnational (regional or municipal) governments. Yet other countries' constitutions, such as those of Germany, India, Mexico, and Nigeria, are federal systems. In federal systems, the constitution grants two or more governments—for example, a state or provincial government as well as the national government— overlapping authority over the same group of people or the same piece of territory. Federal constitutions tend to disperse political power away from the central government, while unitary constitutions tend to concentrate political power in the hands of the national government of the day.
The balance of power between the executive and legislative branches of government varies with different democratic constitutions. The legislature is typically the body charged with making laws, while the executive is responsible for implementing the laws. Presidential systems fragment political power by having voters directly elect a legislature as well as an executive, the president. Examples of presidential systems include Mexico and—when it is operating as a democracy—Nigeria.
In contrast to presidential systems, in which citizens directly elect both the executive and the legislative branches of government, parliamentary systems tend to concentrate political power because voters only elect a legislature. Under this system, the legislature (parliament) itself elects the executive to head the government— a prime minister—from among its members. The UK, Germany, and India are examples of parliamentary systems.
Finally, in semi-presidential systems, voters elect both a president and a legislature, but the legislature then also elects a prime minister, who is accountable both to the parliamentary majority as well as to the president. In such systems, the president and prime minister must share executive authority. France and Russia (if it were to become democratic) are examples of semi-presidential systems. As the name implies, semi-presidential systems combine attributes of both parliamentary and presidential systems. The formal distribution of power in such systems can vary a great deal, but semi-presidential systems tend to concentrate power in the hands of the president.